Obituary. 697 



recting the aberrations of the four crown convex lenses.* From an 

 examination of one of these lenses, a -fa, in the author's possession, 

 it appears that the lens has an initial power of 25 with an N.A. 

 of ■ 65. It is supplied with two stops, which reduce the aperture 

 to 0*52 and 0*35 N.A. respectively. When full aperture is used, 

 some spherical aberration is apparent, but when the 0*52 N.A. 

 stop is inserted the lens performs very well indeed, and for its day 

 it was quite a good glass : in fact, it was only superseded by the 

 advent of the 12 mm. and 8 mm. apochromats. 



In 1876 Wenham published an aperture table giving the N.A. 

 values for dry lenses of various angular apertures, to show that there 

 was not much more to be gained after an angle of 150° had been 

 reached.f 



Wenham's " button " was brought out in the English Mechanic.t 

 This " button," or really half button, consisted of a semicircular 

 disk of glass about \ in. diameter and ^ in. thick. The circular edge 

 was curved at right angles to the plane of the disk, and the flat 

 diameter was oil-immersed to the underneath part of the slip, by 

 which means light of great oblicpuity could be focused on a balsam- 

 mounted object. This, so far as I know, was Wenham's last 

 microscopical inventibn. He published several papers upon the 

 microscopical examination of objects, notably upon the structure of 

 the Podura scale. Wenham was optical adviser to Messrs. 

 T. Ross and Co. for about ten years (1870-80). 



In the above notice the author has given a fairly complete 

 list of Wenham's microscopical inventions, designs, and methods 

 of work, so that the reader may be able to form his own estimate of 

 Wenham's influence upon the progress of " Microscopy." 



Edward M. Nelson. 



* A diagram with traced rays is given in the Monthly Micr. Journ., ix. (1873) 

 p. 163. t Monthly Micr. Journ., xvi. (1876) p. 287. 



X English Mechanic, xxx. (1879). 



Dec. 16th, 1908 3 a 



