499 



Pocket Magnifiers. 

 By G. C. Karop, F.E.M.S. 



) 



{Read March 20fh, 1903.) 



Messrs. Swift have put into my hands for exhibition a pocket- 

 lens which, although possibly not of absolutely novel construction, 

 appears to embody certain very useful features not hitherto, to 

 my knowledge, at least, combined in quite the same way or in 

 precisely the same form as in the instrument now shown. When 

 examining and testing, to some limited extent, the performance of 

 this magnifier in order to be able to say something about its merits, 

 it occurred to me that it might not be unprofitable to myself, and 

 perhaps to a few others if communicated, to look up some of the 

 very numerous devices invented for simple microscopes, or used 

 secondarily as pocket-lenses. Of the equi-convex lens, long the 

 only magnifier and still largely employed, there is little or nothing 

 to be said, except that it is cheap and eflicient enough for rough 

 purposes. Its great aberration, both spherical and chromatic 

 (unless its curves are kept very flat and therefore its power low), 

 is, however, a serious drawback, and one necessarily inherent in 

 all single lenses, although a plano-convex has less aberration, 

 and an inequi-convex with curves of 6 : 1 (in glass of refractive 

 index 1'5) is known as a "lens of least aberration." Neverthe- 

 less, such is our igiiorance or indifference, or perhaps it is merely 

 an expression of the innate and unconscious human hostility to 

 change, that I suppose more than one half of us have, at one 

 time or another, gone about with one, two, or three equi-convex 

 lenses in our pockets, mounted in horn, and most probably made 

 of inferior glass and not worked to an edge. One of the first to 

 attempt an improvement of the simple lens was Dr. Wollaston 

 ("Philosophical Transactions," 1812), who combined two deep 

 plano-convexes by the plane surfaces, with a perforated disc or 

 diaphragm of metal in between. The diaphragm, having a very 

 narrow opening, one-fifth of the focal length, greatly diminiished 



