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Francis Wenham

Courtesy of Larry Albright

Introduction by Larry Albright

One of my virtues and/or vices is an admiration of flawed heroes. One example is Crookes who was fascinated with beautiful gas discharge tubes. He was a pioneer of early photography and in his complaints about fogged film almost, but not quite, was the discoverer of x-rays (which were fogging his film). Then there was Wheatstone, the shy scientist who invented the telegraph (in English history), the concertina, stereoscope and many interesting electrical devices. He did not invent the Wheatstone bridge. Which by way of introduction brings us to Francis Wenham.

The following is excerpted from the Presidential Address by L.V. Martin to the Queckett Microscopical Club (London) in 1973.

Francis Wenham and the Microscope


L.V. Martin

Francis Herbert Wenham was born in Kensington in 1824, the son of an Army surgeon, and as a youngster soon showed an interest in scientific and technical matters. An event that shaped his life was when at the age of 14 he was taken to see the trials on the Thames of a ship, appropriately named the "Archimedes", built to demonstrate the advantages of the screw propeller over the paddles then used. This seems to have decided Wenham to become a marine engineer and specialist in propellers, and when he was 17 he was apprenticed to a company set up in Bristol by the Great Western Railway to build their second Atlantic steamer. He entered the drawing office, no doubt as a premium apprentice since he seems to have been in easy circumstances all his life, and there met many of the leading engineers of the day such as the younger Brunel who designed the ship, and particularly James Nasmyth the inventor of the steam hammer and who was a friend of Wenham for 40 years. The ship being built was the 'Great Britain', the same one which was recently towed as a hulk from the Falkland Islands back to her birthplace in Bristol for restoration.

The portrait (Fig. 1.) is taken from a small card in the possession of the Royal Aeronautical Society by whose kind permission I am able to reproduce it. Wenham's own signature is underneath, written when he was a very old man; I have no idea what the apparatus he holds is, but he seems rather proud of it.

After finishing his time in Bristol, Wenham embarked on a very active and successful career in engineering during which he designed marine engines, ship's propellers, gas and hot air engines, high pressure boilers, a novel road locomotive and many other items. In some cases he manufactured what he designed. Some ships which were fitted with his engines had such a turn of speed that they were used to blockade runners in the American Civil War.

portrait of Francis WenhamFig. 1 Francis Wenham late in life

In his early thirties Wenham took time off from his professional engagements and in a river steamer which he himself had designed and built went up the Nile on a photographic tour with Francis Frith taking pictures of pyramids, tombs, temples and everything else of note including the colossal statues at Abu Simbel. This tour made a lasting impression on Wenham who make frequent references to it later in his writings on aeronautics. He learned much about photography from Frith, and this expedition was the foundation of the latter's fortunes, for the extensive series of photographs which resulted sold well and he eventually built up the largest business in the world publishing travel views. At the end of the trip Wenham sold his boat to the Viceroy of Egypt; I should very much like to see a photograph of it if one exists since it was of unusual design having four propellers driven by straps, and a boiler pressure of 300 lbs. per square inch which was enormously higher than was usual at the time.

Before turning to microscopy I must mention Wenham's principal spare time activity, and the one for which he is now most famous, namely aeronautics. I cannot now go into any detail of his activities in this direction, but Charles Gibbs-Smith, the historian of the aeroplane (that very word coined by Wenham), has written that Wenham exercised a profound influence on the development of flight, and Wilbur Wright in reply to Wenham's congratulations of the Wright brothers' first success referred to him as one of the ablest and most useful men who ever laboured in the cause of human flight. Wenham's contributions to aeronautics would take a good deal longer to cover than his contributions to microscopy. Wenham tells us that he first looked down a microscope when he was 13 years old, but nothing more is heard of him as a microscopist till he was 26 and had already developed a high degree of skill in working glass and metal. From then until he relinquished the microscope some 30 years later he was the author of a very large number of papers on the subject, also of many cognate inventions.

At one time Wenham was noted as an observer as well as an instrument man. He wrote on cyclosis, movement of diatoms and other subjects, but his most notable paper of this sort was 'The Formation and Development of the Vegetable Cell' (1856). His observations seem to controvert the then fairly new doctrines of Schleiden as regards cell division. Dr. Carpenter, who had doubts about current cell theory, was delighted and described the paper as an important rectification of doctrine. Neither he nor Wenham denied that cells could and did multiply by binary division in a growing vegetable, but they held that that was not the only way, and Wenham's observations were confirmed by others.

He entered into controversies over the nature of markings on diatoms and podura scales, and devised special means of preparing and examining them. For instance he heated podura scales on a polished knife blade, 'torrefying' he called it, judging the temperature reached by the oxidasation colours on the surface of the metal, as does a toolmaker when he tempers hardened steel. This is a controlled analogy of the once popular device of charring specimens between glass slips and might be worth trying again,. He made replicas in black wax or by electrotyping of the surfaces of diatoms and other transparent objects so that he could examine them by reflected light without any confusion arising from the structure below the surface. Replicas have been used a great deal in electron microscopy for the opposite purpose, to obtain a transparency of an opaque surface, but Wenham's process might still be useful.

He saw the need for a solid mounting medium other than Canada balsam and concocted a mixture of gelatin and green syrup. This was somewhat coloured but was taken up by Deane, Lawrence and others and developed into the glycerol jelly we know today. Wenham's method of preparing a blowfly's tongue by grasping it between finger and thumb, thus inflating it, and then nipping between two glass slips has been revived from time to time; so also has his simple compressor with thin glasses which he designed especially for his Paraboloid.

The Daguerre photographic process came out in 1839 when Wenham was still a boy but he took an interest in it, and after the much faster wet collodion process was introduced in 1850 he began making photomicrographs. Wenham goes into possible illuminants, including electric sparks from a Leyden jar, but decides on sunlight as the best; he shows how to correct the visual to the actinic focus - a matter of some importance when plates were sensitive only to blue - and indicates how to make stereoscopic micrographs. He describes how he uses a darkened room as his camera with his microscope on a bench against a shuttered window; sunlight is reflected from an outside mirror though a sleeve in the shutter on to the specimen either direct, though a bull's-eye lens or through an achromatic condenser. By occasionally rotating the sleeve and adjusting the mirror by strings he manages without a heliostat. Light for manipulation comes though a yellow glass let into the shutter and he has all his chemical baths by him for processing the plates. An easel which can be moved to and fro along the bench carries a card on which the picture is composed and focused and then the plate is substituted for the card. Wet plates had, of course, to be prepared on the spot for no commercial plates or films could be bought until a dry plate process was achieved a good many years later. The effective inventor of the dry plate was, incidentally, Dr. Richard Maddox a well known microscopist and a friend of Wenham. The frontispiece of the later editions of Beale's 'How to Work with the Microscope' is a photographic reproduction of a number of photomicrographs made by Maddox including several taken with a 1/12-in objective constructed by Wenham.

Since Wenham was actually sitting in his camera while the exposure was proceeding the possibility of shading his plate locally soon occurred to him and he used shaped pieces of paper to adjust the exposure as between dense and transparent parts of the specimen just as a photographer nowadays sometimes shades the bromide paper when enlarging. More than that, Wenham also used the same method for masking off parts of the picture to allow of a different focus, thus getting sharp images of parts lying at different depths.

With the same arrangements but with a camera in place of the microscope he made, possibly, the first-ever photographic enlargements by projection. The idea was not new since it had been suggested previously by Fox Talbot, (a lifelong friend ) but after Wenham had read a paper on 'Enlarged Positives' to the (now Royal) Photographic Society (1853) it was brought out clearly in the subsequent discussion that this was the first time the idea had actually been put into practice.

Apart from designing a friction coarse adjustment he did not concern himself greatly with the microscope stand until he was persuaded to join Ross & Co. as an adviser following the death of Thomas Ross in 1870. He then devised a series of stands as successors of the classic Ross stand contrived by Andrew Ross and improved by his son Thomas. The first of these Wenham stands is illustrated in Fig. 2, taken from the Quarterly Journal of Science (1873); it is a radical departure from its predecessors but was itself soon replaced by another. Wenham wrote a paper in 1875 on the benefits - probably illusory - of oblique vision with high powers, and the second microscope was furnished with a stage which could be tilted in relation to the optical axis, possibly as a consequence. It also had a long lever fine adjustment lifting the whole body, which was more sensitive than the previous one which had a much shorter lever acting on the nosepiece only, and was said to be better for precise micrometry since the distance between the objective and the eyepiece remained constant.

Wenham microscope standFigure 2: The first stand designed by Wenham for Ross and Co.

In the '70's there was a craze for swinging substages to allow very oblique illumination before it was realized that a wide-angled condenser could provide this much more conveniently. This was particularly so in the United States and the model made then by Zentmayer was much admired. Wenham again redesigned the stand to incorporate Zentmayer's substage and since the older foot would not have allowed the substage to swing so readily, he adopted the foot with two turned pillars which was originated by Jackson many years before and used by American opticians. The stand thus became the well-known Ross Zentmayer; several versions of this were produced including a simplified student's instrument.

Finally Wenham produced his magnum opus, the formidable Ross-Wenham Radial. This was brought out in 1882 shortly before he left the firm and was already obsolete by the time it arrived. Nevertheless it was much admired and a number were sold, some with a rather doubtful form of fine adjustment invented by his successor, Dr. Schroeder, who incidentally designed what has been held to be the first anastigmat photographic lens, the Ross Concentric. Whatever the practicability of the enormous Radial it must be one of the most desirable of all collector's pieces in microscopy.

Wheatstone announced his invention of the stereoscope in 1838, but no serious attempt seems to have been made to apply the principle to the microscope - necessarily a binocular - till Riddell in the United States but he improved greatly on this in his following design. The chronology is not clear, but it seems that the first notice of Riddell's binocular did not appear in England till 1853, and then it was his earlier defective instrument that was described. By then Wenham had clearly been working on binoculars for some time, though he never claimed any priority over Riddell, and he published a magnificent paper (1853) concerning the principles on which a stereoscopic binocular should be constructed and making a number of proposals. One of the arrangements he suggested was made for him by Smith and Beck and is illustrated in Beale (1868); in this the tubes are disposed symmetrically unlike the later standard Wenham binocular. Contrary to Riddell's instruments and most other binoculars to this day, this one had a compound refracting prism as the beam splitter - the heart of any binocular microscope - rather than one or more reflecting prisms which do not need to be compound. Fig. 3 illustrates some of Wenham's prisms that were actually made and the one just mentioned is at a; it will be seen that the rays coming from the objective are directed to the eyepieces without being crossed over, which gives a pseudoscopic effect in which elevations in the specimen appear as depressions and vice versa. This could be overcome by using erecting eyepieces each of which would reverse the rays reaching it, but Wenham thought this to be a clumsy expedient and suggested that use might familiarize an observer with the true nature of pseudoscopic images.

prisms used by Wenham for binocular microscope
Fig. 3 A selection of different prisms made by Wenham for the binocular microscope

Thereafter he dropped the subject of binoculars till 1860 when he was persuaded to enter into it again. 'If the pictures in a stereoscope are put in the wrong way round' said he in a paper in that year 'you get a pseudoscopic effect which you correct by reversing the pictures. Let us do the same with the binocular microscope and cross the rays over within the microscope'. And so he did in the prism at b in Fig. 3. This also is a compound crown and flint glass prism; it has to be, otherwise the images would show colour. He said that this prism gave excellent results but it was extremely difficult to make. He himself was about the only man who could have made the four sided element sufficiently truly and he said that it gave him so much trouble that he would not care to make another; although the prism looks fairly large in the diagram it was in fact only as thick as a matchstick. He accordingly redesigned it and the prism at c was the result. This worked as well as the other and several microscopes were made which gave satisfaction to their owners - a rather sort-lived satisfaction I fancy, since at the end of 1860 Wenham published his brilliant design for a reflecting prism which held the field for the next fifty years.

Quite soon after stereoscopic binoculars became available the need was felt for a non-stereoscopic instrument which would not interfere with objective aperture but still afford the comfort of using both eyes. Powell and Lealand brought out a prism for the purpose which was interchangeable with the Wenham prism in their microscopes, but this besides shifting the optical axis only provided about one sixth of the light in one eyepiece that it did in the other. Wenham paid attention to this problem and devised the arrangement shown at d in Fig. 3. Here the rays from the objective are partly reflected and partly transmitted by a very thin layer of air between two adjacent prisms, with those reflected being again reflected by another prism to one side up the auxiliary tube of a standard Wenham binocular. This plan was soon modified as in e to cut out two glass-air surfaces and the reflections arising from them. Rays do not reach the second reflecting surface at a sufficient angle for total reflection, so that surface has to be silvered. This Wenham high-power prism is certainly an improvement over the Powell one but the light is still very unequal as between eyepieces, and this is also the case with the Abbe binocular eyepiece brought out by Zeiss and which copied Wenham's idea of a thin film of air as a beam splitter. At some stage in Ross instruments this air film was replaced by half-silvering or otherwise treating the prisms so that there was equal illumination in each eyepiece, but I cannot say whether this was effected by Wenham or Schroeder, though I suspect it was the later from evidence in another direction.

In all, Wenham designed or suggested no less than 17 binocular arrangements so far as I have been able to trace, including a proposal in his 1853 paper to use a double image prism for a non-stereoscopic binocular. I have it on the authority of Messrs Bausch and Lomb that this is now adopted for certain high-class binoculars, presumably their own.

Wenham first came to notice as a microscopist in 1850 when he produced a paper on darkground illumination. Hitherto this had been unilateral by a lamp or mirror held to one side below the stage or by a substage prism made by Nachet. To secure what was called 'all-round' darkground illumination Wenham devised his silvered reflecting paraboloid which incorporated focusing and centering motions, Fig. 4; one of these was shown on Smith and Beck's stand at the Great Exhibition. Just at this time George Shadbolt, a life-long friend and admirer of Wenham, invented an annular glass illuminator which also appears in Fig. 4, which is taken from the journal of the Microscopical Society (1852). The two joined forces and the result was the Wenham Paraboloid made of glass and much easier to manufacture than the earlier one. The paraboloid became very popular but fell out of favour when wide angled condensers came into common use since these are more convenient for darkground, using patch stops.

Wenham's reflecting parabaloid
Fig. 4 Wenham's reflecting paraboloid and (inset) Shadbolt's glass illuminator

Wenham was very interested in the illumination of opaque objects and devised many schemes for the purpose. He seems to have been the first to use this method, that is to use the objective as its own condenser, and the first to make a vertical illuminator. A principle of illumination he used extensively is shown in Fig. 5. In this the specimen must be in some medium other than air, preferably in balsam. Light is sent into the slide at such an angle that when it reaches the upper surface of the cover glass it is totally reflected down on to the specimen. Wenham went about this in several ways and eventually invented a special reflex illuminator that was much used in the United States. It will be apparent that if the object is in air the rays will be totally reflected by the top surface of the slide itself and will never get the the cover glass. Wenham noted that in these circumstances where the specimen was in optical contact with the slide this total reflection was spoiled and if the specimen was not opaque, light passed into it and it became, as it were, self-luminous. He used this principle to examine podura scales and other difficult objects. Both these methods of illumination might be of value today.

Till Wenham started making objectives, those with correction mounts, and they were much more common than they are now, were constructed with the front element of the combination mounted at the end of a tube which could slide on another tube containing the remaining elements, thus varying its distance from them and compensating for differences in cover glass thickness. The screw adjustment used was inconveniently slow and it was not difficult to put the front lens right through the cover. Wenham arranged for the fronts of his objectives to be fixed and for the rear combinations to move within the objective barrel by the action of a pin in a spiral groove. This construction was adopted by all the makers and is still used today.

Figure 5. Plan for reflection from coverglass

He made his first objective, a 1/4-in., in 1850 and followed this with an 1/8-in. about which he consulted J. J. Lister, who was perhaps second only to Ernst Abbe as an improver of the achromatic microscope. Wenham experimented a great deal with the design of objectives and by 1853 he was making excellent lenses with apertures up to 170° or N.A. 0.99 in today's terminology. Higher than that it was impossible to go until immersion lenses became practicable, as was the case a few years later, although their history goes back at least to Sir David Brewster in 1813.

Wenham was no great mathematician and designed his objectives by tracing rays through diagrams drawn out on a scale of 50:1, a method he learned from J. J. Lister. This worked well enough for him and when he was with Ross and Co. he revised all their high power objectives, giving them a single flint back as well as a single front, leaving only the middle element (or elements) compound. He gave a paper (1872) to the Royal Society on objective design, and Nelson considered some of these Ross objectives to be the best before the advent of apochromatics.

Wenham published a series of articles in the Monthly Microscopical Journal (1869) giving full directions for making objectives and prisms, and some readers did actually make objectives as a result. These articles are well worth consulting since they are full of useful hints on working glass.

So far a success story, but there were grave weaknesses in Wenham's conception of physical optics which were in due course to cost him much of his reputation as a microscopist. The diagrams in Fig. 6 indicate his basic misconception. They both show a cone of rays entering a medium from the air below and contracting in accordance with the laws of refraction, to expand again to the original angle when emerging into the air above. Wenham was convinced that the contraction of the cone represented a loss of aperture never to be recovered and that if an object were mounted in the medium, say balsam, at the position b in the first diagram it could not be seen so advantageously as if it were mounted in air, as for instance where the rays cross in the second diagram. Like most others of his time he confounded angle, or rather the chord of the angle, with aperture, which is really a measure of capacity to transmit radiant energy and is higher in proportion to the refractive index of the medium as well as in proportion to the chord of the angle of transmission. A consequence of this thinking was that no object mounted in balsam could ever be seen under a greater angle than about 82°, or as we would put it now, N.A. 0.65, whatever the aperture of the objective since any additional rays emanating from the object could be totally reflected at the cover glass.

Fig. 6. Diagram showing contraction of illumination cone

Wenham's main antagonist was the American optician Robert Tolles, who with no great literary clarity but with much common sense and practical skill advanced arguments and constructed apparatus and diagrams that eventually showed Wenham's position to be an impossible one. Tolles can probably be regarded as the true progenitor of the homogeneous immersion objective for, no doubt benefiting from Wenham's earlier work, he made in 1872 an objective working in liquid balsam with what was termed a 'balsam angle' of 95°, N.A. 1.12. He claimed, rightly that this was the equivalent of more than 180° in air, which claim Wenham ridiculed on the grounds that no objective could possibly see backwards. This unhappy controversy dragged, with Wenham losing nearly all his support, until a new era in microscopy was heralded in 1873 by the publication of the theories of the microscope arrived at simultaneously by Abbe and von Helmholtz, which cut the ground from under Wenham's feet when they were belatedly made known in England.

That was not the end of Wenham, of course, for he had become interested in gas lighting and took out a number of patents which he exploited with a manufacturing company he set up. His patent shadowless burner became almost universal till the advent of the gas mantle and electric lighting. Having substantial private means he retired from business when he was sixty, though not from technical activities for he later took out patents for a mechanical piano and an infinitely variable gear for motor cars. He continued his active interest in aeronautics and indeed bought ten acres on a hill at Woking to carry out gliding experiments. Even as he died at the age of 84 his last paper on aeronautics was passing through the press.

There is, however, a postscript. Long after the aperture controversy had faded away an article by Wenham appeared in the 'English Mechanic' entitled 'A Microscopical Reminiscence'. In this he describes an experiment to show that objective apertures measured by the usual methods are fallacious and says that a paper he once laid before the Royal Microscopical Society embodying the experiment was peremptorily rejected the Council at the behest of wire-pullers now gone over to the opposition. 'My paper was returned to me' he writes 'without even a request for a demonstration. In consequence of this I tendered my resignation of membership. Directly after this that body of useful workers, the Quekett Microscopical Club, with graceful recognition of my past services for improvements in the microscope and its accessories, at once elected me as a honorary member of their Association'. And, say I, "Good for the Quekett!"

This excerpt is reprinted by permission from Journal of the Microscopical Society of Southern California, February 1998, Gaylord Moss, Editor.

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