£96 Obituary. 



front, a form subsequently adopted. He says : — " A partial ex- 

 periment with a \ having this ' doublet ' front, has proved that 

 perfect correction for colour is the result. But in the form tried, 

 the spherical aberration was so considerable as to require an 

 entire reconstruction, for which I have now no leisure ; and 

 though the entire success of the idea is yet unproved, I venture 

 to record it, in case I may never be able to take up this subject 

 again, as I am of opinion that a very perfect object-glass may be 

 made of this form." 



Wenham claimed to be the inventor in 1850 of the single 

 front ; this invention has been also ascribed to Professor Amici, 

 but I am not aware of any published statement to upset Wenham's 

 claim. All improvements in Microscope object-glasses were trade 

 secrets, and it is at the present time a matter of no little difficulty 

 to find out their true history. 



In 1871 we find the aperture controversy still raging. Wenham 

 says : — " The loss of aperture on balsam-mounted objects was 

 demonstrated by me on correct optical laws known ages ago, and 

 I am astonished that in the nineteenth century anyone can dare to 

 dispute it as a fact." 



Wenham was not, and never claimed to be, either a mathe- 

 matician or a physicist. He explains his method of designing 

 object-glasses by means of graphic delineation and of trial and 

 error, which he had learnt from the personal teaching of Mr. J. J. 

 Lister. The paths of the various rays were plotted down on an 

 enlarged diagram of the object-glass ; the sines of the angles of 

 incidence and refraction were placed upon a large pair of pro- 

 portional compasses, set to the index of refraction. This probably 

 was the method employed Ify all Microscope object-glass makers at 

 that time.* 



Plotting may do fairly well for marginal rays, but with axial 

 rays it fails altogether, and, so far as I am aware, axial rays can 

 only be dealt with by trigonometry, or by the Gauss method, both 

 of which were probably beyond Wenham's mathematical capabilities. 



This lack of familiarity with elementary mathematics was a 

 cloud which obscured his vision on many important points ; if only 

 it could have been lifted, what an inventor he would have been ! 

 For instance, the mere elements of algebra would have enabled him 

 to turn his " duplex " front, which by itself is unimportant, into the 

 " aplanatic " front, the main feature of the modern object-glass. 



In 1872 his reflex illuminator appeared ; this was a kind of 

 immersion Nachet prism. It did not come into very general use, 

 because it was soon superseded by immersion condensers. 



In 1873 Wenham brought out an objective upon a new formula 

 — a single front and back and a triple middle ; a single flint cor- 



* Wenham gives an interesting example in the Monthly Micr. Journ., v. 

 ■41871) p. 19, fig. 1. 



