CONSTRUCTION OF THE MICROSCOPE. 33 



nearer the margin of the second lens, where the refraction, 

 being more powerful than near the centre, compensates in 

 some degi'eo 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 diminished 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 compensa- 

 tion would take place, quite different in principle, and 

 inferior in degree, to that which is produced in the excen- 

 trical 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 micro- 

 scope 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 

 t^us effected by two lenses instead of one, is accompanied 

 by smaller aberrations, which are, therefore, more com- 

 pletely balanced or corrected at the second bending, in the 

 opposite direction, by the third lens. 



Hand Magnifiers, — Before we proceed further, it will 

 be as well to bestow a passing notice on the simple hand 

 magnifier, so often employed by microscopists in the pre- 

 liminary examinations of objects. 



A very good form of lens was proposed by Dr. Wollaston, 

 and called by him the Periscopic lens : which consisted of 



