204 Aiuericaji Quarterly Microscopical Journal. 



more than half the light is reflected from the front surface, with- 

 out entering the lens at all, and yet more at higher obliquities. 

 I do not mean to say that there is not a gain worth striving for 

 in passing from — say i6o° to 170°, but an objective, whose real 

 air angle is more than that last named, will have an inconveni- 

 ently short working distance. I intend to investigate the cause 

 of this enlarged angle of dry objectives observed in the Abbe 

 apertometer fashion, with the bull's eye. I find that the 

 Spencer Student ^th inch, which measures 100° with the glass 

 slide and lines, or by looking through the eye-hole at b, as 

 above described, gives, by the latter mode, when the bull's 

 eye is interposed, 6-^° as angle in glass, from which the com- 

 puted air angle will be near 104°. Using the bull's eye, slide, 

 and lines as directed, and looking through b the glass angle is 

 61°. 5, from which the computed air angle is 100°. With object- 

 ives of shorter focal strength the agreement is closer. 



I do not yet see any need of adopting Dr. Abbe's notation, 

 which is, indeed, strictly correct in theory, but it would be very 

 inconvenient for many microscopists, as it necessitates the use 

 of tables that some might not have, or others understand. 

 The fact is, no objective can be made to do equally as good 

 work dry and immersion; it should be either dry or immer- 

 sion simply. Why not mark the angle just what it is ? If the 

 objective is an immersion of 116', everybody will understand 

 it, and none but a simpleton would confound it with 116° 

 marked on a dry lens, or suppose that it meant an objective of 

 the same resolving power, or, indeed, excellence anyway. 

 Knowing that 82° on an immersion would be equivalent to 

 180° on a dry, everyone would soon learn the comparative 

 values of the same angles marked on each, and neither would 

 ever come dangerously near 180". 



To compute rigidly the balsam or glass angle from the ob- 

 served air angle, or vice versa., the objective must not only be 

 acccurately focused, but the intersecting lines must be used; 

 otherwise an exaggerated angle (varying very much in differ- 

 ent objectives) to the extent of two to twelve degrees, may be 

 obtained as the glass angle; and if, from this, we should 

 compute the air angle, it might be ten or twenty degrees too 

 much. 



Carefully used, the instrument will give entirely concordant 

 results. Moving to the right, the point of intersection will 



