38 THE MICROSCOPE. 



nearly parallel, and of consequence colourless to the 

 eye. At the same time the spherical aberration has 

 much diminished, "because the side of the pencil as it 

 proceeds through one lens passes nearest the axis and in 

 the other nearest the margin. 



This explanation applies to pencils farthest from 

 the centre of the object. The central pencils, it is 

 obvious, would pass both lenses symmetrically, tho 

 same portions of rays occupying nearly the same 

 relative places in both lenses. The blue ray 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. 



The next step towards improving the simple micro- 

 scope was in relation to the eye-piece ; this was effected 

 by Mr. Holland. It consists in substituting two lenses 

 for the first in the doublet, and placing a stop between 

 them and the third. The first bending of the pencils 

 of light being effected by two lenses instead of one, 

 produces less aberration, and this is more completely 

 balanced or corrected at the second bending, and in 

 the opposite direction, by the third lens. 



A useful form of pocket lens was proposed by Dr. 

 Wollaston, named by him " Periscopic." This combi- 

 nation consists of two hemispherical lenses cemented 

 together by their plane faces, with a stop between 

 them to limit the aperture. A similar proposal, made 

 by Sir David Brewster in 1820, is well known as the 

 Coddington lens, 1 shown at fig. 23 : this gives a 



(1) The late Mr. Coddington, of Cambridge, who had a high opinion of the 

 value of this lens, had one of these grooved spheres executed by Mr. Carey, 

 who gave it the name of the Coddington Lens, supposing that it was invented 

 by the person who employed him, whereas Mr. Coddington never laid claim 

 to it, and the circumstance of his having one made was not until nine years 

 after it was described by Sir David Brewster in the " Edinburgh Journal." 



