500 



G. C. KAROP ON POCKET MAGNIFIERS. 



the spherical abeiTation (the light also) ; and from the oblique 

 rays, like the central, meeting the surface at a right angle in 

 practically whichever way the lens be held, it was called a 

 " periscopic." Brewster, however, pointed out that to make it 

 really efficient the interval betw^een the lenses should be filled by 

 some substance, such as balsam, nearly approaching the refractive 

 index of the glass. Somewhat later, Brewster conceived the idea 

 of arriving at the same end more easily and effectually by 

 grinding a shallow, subcylindrical groove round the equator of a 

 glass sphere, and so doing a way with both cement and diaphragm. 

 It is doubtful, however, whether, except in simplicity of construc- 

 tion, this was any very great advance on Wollaston's lens, as, 

 although two surfaces of refraction were eliminated, the amount 



Wollaston (1812) 



Brewster. 



Coddington (1830). 



of marginal error was but little reduced by the shallowness of the 

 groove at first employed. Coddington ("Philosophical Transac- 

 tions," 1830) proposed that the groove should be made both deeper 

 and of angular form, and this proved so much more advantageous 

 that it held its ground as a pocket magnifier until the advent of 

 the achromatic and aplanatic triplet of Steinheil, and is still, 

 I suppose, a purchasable article. This lens was also made with 

 a narrow channel instead of an angular groove, which, of course, 

 answers equally well. From its form, resembling two truncated 

 cones, apex to apex, it was called a " coneopside," or, as it also 

 has a likeness to the eye of certain birds, "a bird's-eye lens"; 

 but Coddington's name has always been associated with it, and 

 in my humble opinion very properly, as it is a distinct improve- 

 ment on Brewster's original idea. Its chief drawback is short- 

 ness of focus, and this defect is still greater in the so-called 

 " Stanhope " lens, which at one time was sold as a cheap, and 



