694 Obituary. 



receives the light from the Luminous source, and one image will 



disappear ; on reversing the card, so as to cut off the other 

 extreme, the first image will reappear again, and the second 

 vanish." From this abstract we learn that Wenham thus early 

 originates the celebrated Abbe experiment of cutting out first the 

 inclined " dioptric beam," and then the first '"' diffraction spectrum." 

 If Wenham had only assigned the origin of the phenomenon to 

 diffraction, and the overlying of the image to spherical aberration, 

 he would have anticipated much that was to follow twenty years 

 later. 



On page 150 he describes a method of obtaining a dark ground 

 by cutting out the central rays by a stop placed in the object-glass, 

 a device which has recently been re-invented. In the same year 

 (1854) he designs the method of moving by the correction-collar 

 the back lenses of an object-glass instead of the front, a plan 

 now universally adopted. At that time there was a discussion in 

 progress about apertometers, with reference to one designed by the 

 learned Dr. Kobinson (which consisted in illuminating the object- 

 glass through the back, and of measuring the diameter of the disk 

 of light projected upon a card held in front, from which data the 

 tangent of half the angular aperture could be found). Wenham 

 placed a block of glass ^-in. thick, having one side coated by a 

 thin film of bees'-wax. The object-glass to be measured was 

 focused upon the clear side of the glass block, and the disk of 

 light received upon the bees'-wax. The angle in glass was then 

 measured. The following were the results he obtained: — "A 

 j^ having an aperture of 146° on an object mounted dry, was 

 reduced to 75° on an object in balsam ; a \ of 125° to 71°; a ^ of 

 105° to 68° ; and a T 4 D of 90° to 56°. . . . These experiments will 

 readily account for the difficulty of discovering the markings or 

 structure of a severe test when mounted in balsam ; for, as thus 

 seen, it may be inferred that no aperture exceeding 85° can be 

 made to bear upon it, and this is even supposing that the largest 

 aperture object-giass that has ever been constructed is used." 

 This lather long extract is inserted to show that Wenham was 

 the first to measure the aperture of an object-glass with a glass 

 apertometer, and also to acquaint the reader with the general 

 trend of the argument, which runs through his numerous writings, 

 viz. that resolution is due to the angular inclination of the beam 

 proceeding from the object with the optic axis. Wenham argues 

 that there is a loss in a balsam mount because the angle of the ray 

 proceeding frcm the object is limited to 41°, whereas in a dry 

 objective it may be 75° or 80°. This was the beginning of the 

 celebrated aperture controversy, which eventually caused his 

 rupture with this Society in 1879. The heated discussions, and 

 amount of personal feeling brought into them, will be remembered 

 by some, and regretted by all. Before dismissing this painful 



