13 



made of plate glass of 1'5 index of refraction ; the first lens 

 being equiconvex, and the second concavo-convex, whose 

 radii are as 1 to 101 — which combination for forming an 

 image of a distant object — as a silvered-glass ball or illu- 

 minated globe or lamp— is absolutely aplanatic for plate glass 

 of refractive power 1-5. I propose to name this instrument 

 the aplanatometer. 



Armed with such an analyser amateurs will experience 

 little difficulty in examining the performance of their 

 objectives. The appearance of black points on a white disc, 

 or a white disc upon a black ground, as imaged by 

 minute lenses, affords one of the most instructive lessons in 

 microscopic aberration to students of this difficult subject. 

 Familiarity with these optical phenomena of hiown objects 

 enables the observer to detect the errors of observation with 

 fidelity. 



When a thorough acquaintance has been made with these 

 signs of defective correction, objectives required to be ex- 

 amined wall be found either to increase or diminish the 

 glaring aberration of these minute images, and increase, or 

 even destroy, the penumbra of circular aberration. 



Still more difficult of definition is the minute lenticular 

 image of a brilliant point, or of a minute brilliant aperture, 

 or reflecting mercurial globule, placed sufficiently far from 

 the lens to produce a minute image. Thus, if the lens is 

 the 20th of an inch in focal length, the image of the globule 

 will become smaller and smaller as it recedes. 



At a distance of forty inches the image of a globule of 

 mercury 100th of an inch in diameter will be 80,000th of 

 an inch + 64,000,000th, very nearly. By this plan, there- 

 fore, we possess a means of forming images of known dia- 

 meter, and with more or less aberration, according to the 

 kind of lenses employed. 



The Podura is, according to the high-class objectives made 

 by Messrs. Powell and Lealand, a scale covered with ribbing 

 on each side of the basic membrane ; each ribbing is separated 

 by a clear interval, through which the second set of beads 

 placed on the other side are visible. When the scale is 

 folded over, so as to expose the underneath side, a similar 

 ribbing is seen. INIr. Wenham states^ that he cannot see any 

 signs of structure at the folded edge, but the ribs and intervals 

 are clearly displayed when the objective corrections are 

 exquisitely performed. These ribs are composed of beads, 

 varying from the 30,000th to the 150,000th of an inch in 

 diameter, according to the size and character of the scale. 



' 'Microscopical Trausac.,' July, 1869. 



