60 W. LeConte Stevens — Microscope Magnification. 



This formula is fully worthy of reliance if accurate values 

 of the equivalent focal length of eye-piece and objective, 

 respectively, are stamped on their mountings, and if the tube- 

 length also is stamped on the microscope body. 



But the difficulty of securing definiteness and uniformity in 

 tube length is probably greater than that of securing proper 

 labels on the mountings of the lenses. It is necessary to fix 

 upon two points of the microscope body as the upper and 

 lower limits of the tube-length, and additionally for some 

 agreement to be reached among makers as to the tube-length 

 selected. What this shall be is a matter partly of precedent, 

 partly of convenience. The nominal standard is 10 inches in 

 England and America, but there is no pretense of adhering to 

 it. In Germany and the continent of Europe generally, about 

 180 millimeters is perhaps most common. The latter is for 

 some reasons more convenient, and seems to be gaining in 

 popularity. 



From what has preceded it is obvious that the upper limit 

 of the tube-length should be the focal plane in which an image 

 would be formed by the objective if no field lens were inter- 

 posed. If the eye-piece is made to fulfil the generally 

 assumed condition that the focal length of the field lens shall 

 be three times that of the eye lens, and the interval between 

 them shall be twice the focal length of the eye lens, the focal 

 plane in question would be just midway between the diaphragm 

 of a negative eye-piece and the optical center of the eye lens, 

 which is at the middle of its convex surface. The eye-piece 

 should be so constructed that when it is slipped into position 

 this focal plane shall be exactly at the top of the microscope 

 body, which then serves always as the upper limit of tube- 

 length. The desirability of making all eye-pieces thus " par- 

 focal " has been already suggested by several writers. There 

 is no practical mechanical difficulty in attaining this end. In 

 case the negative eye-piece should not fulfil the generally 

 assumed conditions, the distance of the parfocal plane above 

 the diaphragm is easily found. Referring to Fig. 2, and using 

 the same notation, this distance is P'P, or p—p', which} from 



111. v' % 



the formula = — , is equal to -4 — — ,• The required 



p> p /"' * j"-p> 



distance of parfocal plane above diaphragm is thus given in 

 terms of the focal length (/'") of the field lens and the dis- 

 tance {p') of the diaphragm from the optical center of this lens. 

 It should in justice be mentioned in this connection that at 

 least one celebrated European firm, that of Carl Zeiss, in Jena, 

 has for several years past been making all of its eye-pieces par- 

 focal. This is only one of the many good things for which 

 the scientific world is indebted to Professor E. Abbe, a phys- 



