W. LeConte Stevens — Microscope Magnification. 51 



By some authors a distinction is made between the terms 

 r < magnification" and "amplification," and still further be- 

 tween " relative," " comparative," and " absolute " amplifying 

 power.* Whatever may be the value of these distinctions in 

 theory the writer can find no good reason for discarding the 

 familiar term, magnification, to denote the ratio of the diam- 

 eters of the retinal images produced with and without the 

 magnifying lens, or system of lenses, respectively. The con- 

 ditions under which the magnifying system is employed are to 

 some extent arbitrary. 



To compute the magnification given by a microscope it is 

 necessary to multiply together the separate magnifications due 

 to the eye-piece and objective employed. Unfortunately the 

 nomenclature of eye-pieces and objectives is still far from sat- 

 isfactory ; and it would perhaps be safe to say that the 

 majority of persons who employ them are unable, under exist- 

 ing limitations, to do more than accept certain labels and use 

 these in calculation. But the labels are misleading. To call 

 an eye-piece " shallow " or " deep," or to name it an A, B, or 

 C eye-piece, affords no definite idea of its power. Such arbi- 

 trary and useless designations deserve to be abolished. An 

 eye-piece should be labeled with its equivalent focal length 

 like an objective ; and in each case the label should be accu- 

 rate to within one millimeter. This method of labeling eye- 

 pieces was recommended several years ago by the American 

 Society of Microscopists, but thus far there has been very 

 little compliance on the part of manufacturers. Tables of 

 magnification are given by certain firms for combinations of 

 objectives with eye-pieces as sold by them; but the purchaser 

 has to take these figures on trust. They are professedly 

 applicable only when "standard tube-length" is employed. 

 Such a standard exists only in name and not in fact. In 1887 

 Professor S. H. Gage, of Cornell University, applied to all of 

 the prominent makers of microscopes in the world for infor- 

 mation as to the tube-length for which their objectives were 

 corrected, enclosing to each a diagram upon which should be 

 marked those points on the microscope body which were taken 

 as the limits of tube-length. From eighteen of these firms, in- 

 cluding the majority of those addressed, satisfactory answers 

 were obtained. Among the lengths given, the following in 

 millimeters may be taken as examples : 125, 146, 150, 160, 

 165, 180, 190, 200, 203, 216, 220, 228, 250, 251. The 

 last of these numbers occurs most frequently, corresponding to 

 10 inches. Examination of the diagrams revealed equal 

 diversity in regard to the points taken as the limits of tube 

 length. In one case it was from the upper surface of the eye 

 lens to the lower extremity of the objective ; in another, from 



* L. Didelot, "Du Pouvoir amplifiant du microscope," Paris, 1887. 



