154 



THE AMERICAN MONTHLY 



[iVugust, 



nine months since, and, though left 

 in the tube in which they were stain- 

 ed, they are in every particular as 

 perfect as at the time they were pre- 

 pared. After staining, the casts can 

 be mounted in balsam or dammar 

 without undergoing any change. 



A. T. Parker. 



Cabinet for Slides. 



BY J. H. PILLSBURY. 



I have used the following with a 

 great deal of convenience : — 



In order that I might arrange my 

 slides systematically, they must all lie 

 flat to accommodate those which 

 would be injured in any other posi- 

 tion. I had neat trays with sawed 

 slots for twenty-five slides in each 

 tray, arranged on end in a case, with a 

 lid about two inches deep to allow the 

 trays to project far enough to be 

 taken out easily when the lid is open. 

 Each case holds twenty trays in two 

 rows, accommodating five hundred 

 slides. Labels for the names of the 

 slides are stuck on the upper ends 

 of the trays, and the slides may be 

 numbered and lettered to correspond 

 with letters on the trays, and num- 

 bers on the slots if desired. When 

 the lid is open I have a classified 

 list of the five hundred slides before 

 me for instant reference. 



Springfield, Mass. 



Significant Angle. 



In the April number of this Jour- 

 nal, appeared a paper on Telescopic 

 Field and Microscopic Aperture. 



The author of that paper, the Hon. 

 J. D. Cox, thinks the angle usually 

 called the angle of aperture of 

 microscopic objectives should give 

 place to an entirely different 

 angle, measured by a method 

 which he clearly describes. Of this 

 proposed angle he says, on page 64 : 

 " If there be any significant angle in 

 optics, it would seem to be this, and 

 it would seem also to be the true an- 

 gle of aperture of the lens, if the term 



has any definite meaning." Devotees 

 of microscopy will naturally examine 

 pretty carefully any angle which is 

 thus offered as a substitute for the 

 present one, however willing to adopt 

 any improvement. 



The precise difference between the 

 accepted angle of aperture and the 

 one under examination is this: the 

 former is bounded by rays which pro- 

 ceed from a point of the object, situ- 

 ated on the principal axis, and pass 

 through opposite margins of the ob- 

 jective to a conjugate point of the 

 image; the latter is bounded by rays 

 proceeding from a point between the 

 object and the lens, passing through 

 opposite margins of the lens to oppo- 

 site margins of the field-glass of an 

 ocular. 



The angles under consideration are 

 I o I' and / o" I' of fig. 24, page 63, to 

 which figure the reader is referred. 



Inspection of that diagram will en- 

 able any one to see that I l' \% " sig- 

 nificant " to the extent that it gives 

 the angular breadth of the cone of 

 rays which the lens is capable of trans- 

 mitting — theoretically at least — from 

 a point of the object to a conjugate 

 point of the image; and that the vari- 

 ation of the angle, as the distance of 

 the object from the lens is changed, 

 corresponds to the variation of the 

 efficiency of the lens used with differ- 

 ent lengths of tube. Indeed, if the 

 angle and the tube-length with which 

 it was measured be given, it is easy 

 to say in advance what will be the 

 performance of the lens with any tube- 

 length; and the eye-piece employed is 

 a matter of indifference except as to 

 the question of magnifying power and 

 amount of image visible. 



The angle / <? /' is "significant," 

 again, because it, supplemented by 

 the focal length, enables us to deter- 

 mine the effective linear aperture of 

 the lens, which gives it special claim 

 to be called the angle of aperture. 



Moreover, since the vertex is al- 

 ways situated in the object, / o /' is 

 usually regarded as the " angle of 

 aperture " for all applications of the 



