G. C. KAROP ON POCKET MAGNIFIERS. 501 



bad, substitute for the Cotldington. This consisted of a short 

 glass cylinder having two unequal curvatures worked at its ends, 

 and so calculated that, the greater being held next the eye, 

 objects lying on the lesser were in focus. The inferior curvature 

 was provided to avoid distortion of the field, but in the cheapest 

 and worst form, known as a Stanhoscope, and sokl by hawkers in 

 the street, it is more or less a plane, and useless except as a toy. 



Next we come to combinations of two or more lenses, super- 

 posed with a view of partially balancing their individual aberra- 

 tions, which, however, in a more or less unscientific manner, had 

 been in use long before. Divini (" Philosophical Transactions," 

 1668) combined two plano-convexes with their curves in apposition, 

 and Joblot two biconvexes, and also pianos fitted in a sliding 

 tube, so as to obtain varying magnifications. Adams, according 

 to Harting, who gives a figure, made piano doublets in a fixed 

 mount, as also did Frauenhofer in later times, and this con- 

 struction is not to be despised. As a boy I possessed one made 

 by an optician in Leipzig, and prized it greatly for many years, 

 until one day it was borrowed by a barrister to examine a 

 signature on some deed, and, like the little kitten in the poem, 

 " ne'er was seen again." 



Euler was, I believe, the first (in 1764) to work oat the 

 theoretical curves for a true optical doublet, which consisted 

 of a biconvex and a meniscus, the distance between them varying 

 according to the focus ; but I rather think this was intended for 

 telescopes, and it seems doubtful if any were ever made. The 

 elder Herschel took up the matter again in 1821, and worked 

 out formulae for several doublet combinations. One is the same 

 as Euler 's, except that the convex surfaces are in contact, and 

 the focus therefore unalterable ; another consists of two plano- 

 convexes of unequal curvature, with foci as 1 : 2-3, also in contact 

 as in Divini's ; and a third of a crossed lens of least aberration, 

 6 : 1, combined with a plano-convex whose focus is to the former 

 as 2*6 : 1. This last is still used, and gives excellent results ; 

 indeed, the instrument before you is based on a modification of it, 

 the piano having a much smaller radius of curvature. It should 

 be said, however, that these doublets of Herschel were not 

 intended for low-power hand magnifiers, but as objectives, and 

 the difficulty of constructing relatively high powers with the 

 calculated curvatures proved, at that time, insuperable, and they 



