Ai 



CONSTRUCTION OF THE MICROSCOPE. 



two hemispherical lenses cemented together by their plane 

 faces, having a stop between them to limit the aperture. 

 A similar proposal was made by Sir David Brewster in 

 1820, who, however, executed the project in a better man- 

 ner, by cutting a groove in a whole sphere, and filling the 

 groove with opaque matter. His lens, which is better 

 known as the Coddington lens, 1 is shown at fig. 21 : it 

 gives a large field of view, which is equally good in all 

 directions, as it is evident that the pencils ab and ba pass 



Fig. 21. 



Fig. 22 



through under precisely the same circumstances. Its 

 spherical form has the further advantage of rendering the 

 position in which it is held of comparatively little conse- 

 quence. It is therefore very convenient as a hand magni- 

 fier ; but its definition is, of course, not so good as that of 

 a well-made doublet or achromatic lens. It is generally 

 set in a folding case, as represented in the figure, and so 

 contrived that it is admirably adapted for the waistcoat- 

 pocket ; which, together with the small holder, fig. 22, for 



(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." 



