176 



THE AMERICAN MONTHLY 



[September, 



Secretary, D. S. Kellicott ; Treasurer, 

 George E. Fell ; Executive Com- 

 mittee, H. F. Atwood, L. M. East- 

 man, F, S. Newcomer. 



Several dealers in microscopical 

 goods were represented at the meet- 

 ing. Mr. Edward Pennock had a 

 large number of instruments from J. 

 W. Queen & Co., including a set of 

 Prof. Abbe's diffraction plates and 

 diaphragms, to illustrate his theory 

 of vision with the microscope ; an 

 Abbe test-plate for studying the cor- 

 rection of an objective ; a mechani- 

 cal finger devised by Prof. C. H. Kain, 

 and various other articles of interest. 



Messrs. Bausch & Lomb had a 

 number of their excellent micro- 

 scopes and objectives on exhibition, 

 in charge of Mr. Edward Bausch. 

 Mr. W. H. Walmsley exhibited a line 

 of goods by R. & J. Beck, of London. 

 Mr. L. R. Sexton had with him some 

 of Mr. Gundlach's latest productions, 

 but an attack of sickness prevented 

 him from showing them. 



A Simple Method of Determin- 

 ing the Angle of Aperture of 

 Immersion Objectives. 



For the determination of the angu- 

 lar aperture of objectives, if not less 

 than 96° in crown-glass, I propose to 

 attach to the front surface of the objec- 

 tive, by means of a "homogeneous" 

 medium, in the usual way, a small 

 piece of crown-glass, which has, be- 

 sides the adhering surface, two other 

 polished plane surfaces, at right an- 

 gles to the former and parallel to 

 each other, with a distance between 

 them of at least the diameter of the 

 front lens of the objective. 



Then, from two distant points, ly- 

 ing in the plane described by the op- 

 tical axis of the objective and the 

 perpendicular upon this axis and the 

 parallel plane surfaces of the glass 

 piece, let rays of light fall upon these 

 surfaces, to pass through the glass and 

 then through the objective. 



Find, in the usual manner, by mov- 

 ing the lights sideways, that direction 



of the two light rays by which the 

 latter will just strike the outer edge 

 of the aperture of the objective. 

 Then determine the angle described 

 by the two rays before entering the 

 glass piece, and find the true crown- 

 glass angle of the objective by calcu- 

 lation after this formula : — 



COS. i 



~*^-=-cos. a 

 r 



i being half the angle of the two 



rays before entering the glass piece ; 



r, the refractive index of the glass 



piece ; a, half the crown-glass angle 



of the objective. 



Ernst Gundlach. 



EDITORIAL. 



_ Subscriptions. — Remittances for subscrip- 

 tions should be made by post-office or express money- 

 orders, by draft payable in New York, or in registered 

 letters. Money sent in any other way will be at the 

 sender's risk. A receipt will bs immediately given for 

 money received by open mail. 



The Journal is issued on the isth day of each 

 month. Subicribers who do not receive their copies 

 at the usual time are requested to inform the Pub- 

 lisher of the fact. 



The August Meetings. — We have 

 given up the greater part of the Jour- 

 nal this month to reports of the 

 August scientific meetings, deeming 

 it on the whole more satisfactory to 

 print these reports while they are 

 fresh and new, than to continue them 

 through two numbers, as heretofore. 

 The meeting at Elmira was, we are 

 informed, a great success, and has 

 doubtless given a great impetus to 

 the Society. It has also shown that 

 the main support of the Society is 

 from the West. All the officers elect- 

 ed are Western men, and it has been 

 stated that only one member from 

 east of the Hudson River was present 

 at the meeting. The Society should 

 not fail to recognize how much is due 

 to the efforts of a few gentlemen in 

 the Elmira Society for the success of 

 this meeting. 



It must be a cause of sincere re- 

 gret to those members of the Society 

 who are aware of the facts in the 

 case, that one gentleman who was 



