Obituary. 695 



subject, it is greatly to Wenham's credit that, although his 

 theory was physically wrong, and could not be maintained for a 

 moment, his arguments and rejoinders were set forth in temperate 

 language — even when invective and abuse were thrown at him 

 by those who did not possess a tithe of his knowledge on micro- 

 scopical optics, but were mere babblers of an elementary text-book, 

 which they probably did not comprehend. 



In 1855 he made electrotype casts of diatoms, and invented the 

 plan of placing a biconvex lens at the back of an object-glass to 

 correct for the actinic focus. So early as 1855* we find him ex- 

 perimenting with homogeneous immersion. He says : — " I first 

 took a small hemispherical lens of about jfe inch radius, and 

 cemented it over a selected specimen of one of the Diatomacea? 

 (N. sigma) with Canada balsam. ... It will be seen from the 

 position of the object, that each ray of light passing from that point 

 through the surface of the hemisphere, will be transmitted in 

 straight lines, in a radial direction, without undergoing any refrac- 

 tion ; the consecpience of which is, that the full and undiminished 

 aperture of the object-glass is made to bear upon the object." He 

 placed the diatom, with this hemispherical lens optically connected 

 with it, underneath a dry achromatic object-glass, and so viewed 

 the object through a homogeneous immersion system. He notes 

 the advantage of homogeneous immersion thus : — " I have a 

 specimen of P. formosum, mounted in this manner, by which the 

 markings are remarkably well displayed." He suggests that the 

 substage condenser should also be made homogeneous. Although 

 Professor Amici in 1844 experimented with an oil- immersion, 

 Wenham was the first to use one in this country, and probably it 

 was Wenham's writings that drew the*attention of Tolles to the 

 importance of this subject, and caused him to make Iris balsam- 

 immersion objective. It is to Tolles that we are indebted for 

 applying the word " homogereous " to the immersion principle. 



In the Transactions of this Society in 1856 we find a paper where 

 Wenham describes several methods for illuminating microscopical 

 objects by immersion condensers, the fluid used being oil of cloves. 

 The principle here laid down, viz. that of total reflection from the 

 inside of the cover-glass, has been lately re-invented. 



In 1859 he designed a tank Microscope, and suggested friction 

 gearing : within recent years his suggestion has been applied to^ 

 stage movements with success. 



Wenham was a very expert mechanic ; he not only designed 

 object-glasses, but made them, grinding and polishing the most 

 minute lenses. In the Monthly Microscopical Journal, i. and ii. 

 (1869) he published five valuable papers upon the practical con- 

 struction of Microscope object-g lasses, written from the standpoint 

 of an amateur. In one qf these papers he proposed the duplex 



* Quart. Journ. Mior. Sci., iii. (1855) p. 303. 



