CONSTRUCTION OF THE MICROSCOPE. 25 



allusion to a stop, nor is it certain that he contemplated its introduction ; 

 although his illness, which terminated fatally soon after the presenta- 

 tion of his paper to the Royal Society, may account for the omission. 



The nature of the corrections which take place in the doublet is 

 explained in the annexed diagram, where lol' is the object, p a portion 

 of the pupil, and dd the stop, or limiting aperture. 



Now it will be observed that each of the pencils of light from 

 the extremities IV of the object is rendered excentrical by the stop; 

 consequently each passes through the two lenses on opposite sides of 

 their common axis op; thus each becomes affected by opposite errors, 

 which to some extent balance and correct each other. To take the 

 pencil I, for instance, which enters the eye at r 6, r b : it is bent to the 

 right at the first lens, and to the left at the second ; and as each bending 

 alters the direction of the blue rays more than the red, and moreover 

 as the blue rays fall nearer the margin of the second lens, where the 

 refraction, being more powerful than near the centre, compensates in 

 some degree for the greater focal length of the second lens, the blue 

 rays will emerge very nearly parallel, and of consequence colourless 

 to the eye. At the same time the spherical aberration has been dimin- 

 ished by the circumstance that the side of the pencil which passes one 

 lens nearest the axis passes the other nearest the margin. 



This explanation applies only to the pencils near the extremities of 

 the object. The central pencils, it is obvious, would pass both lenses 

 symmetrically, the same portions of light occupying nearly the same 

 relative places on both lenses. The blue light would enter the second 

 lens nearer to its axis than the red ; and being thus less refracted than 

 the red by the second lens, a small amount of compensation would take 

 place, quite different in principle, and inferior in degree, to that which 

 is produced in the excentrical pencils. 



In the intermediate spaces the corrections are still more imperfect 

 and uncertain ; and this explains the cause of the aberrations which 

 must of necessity exist even in the best-made doublet. It is, however, 

 infinitely superior to a single lens, and will transmit a pencil of an 

 angle of from 35 to 50 without any very sensible errors. It exhibits, 

 therefore, many of the usual test-objects in a very beautiful manner. 



The next step in the improvement of the simple microscope bears 

 more relation to the eye-piece ; this was effected by Mr. Holland : it 

 consists in substituting two lenses for the first in the doublet, and 

 retaining the stop between them and the third. The first bending 

 being thus effected by two lenses instead of one, is accompanied by 

 smaller aberrations, which are, therefore, more completely balanced 



