PHILOSOPHICAL SOCIETY OP WASHINGTON. 19 



the "balsam angle" of the objective. Subsequently, at the sug- 

 gestion of Mr. Tolles, of Boston, he substituted for the sheet of 

 balsam a quadrangle of crown-glass ground on one side and pol- 

 ished on the edges. One of the edges was placed in front of the 

 objective with which it was connected with the immersion fluid 

 for which the objective was intended. The solar rays coming 

 from behind, after passing through the crown-glass front of the 

 objective, and suffering refraction on entering and leaving the 

 immersion fluid, resumed in the quadrangle of crown-glass the 

 direction they had in the crown-glass front. The margins of the 

 visible cone were marked on the ground glass surface of the quad- 

 rangle with a lead-pencil, and the angle read by a protractor. The 

 angle thus measured was of coui'se substantially the angle of the 

 extreme rays in the crown-glass front. It might be conveniently 

 called the " interior angle" of the objective, with which it would 

 be identical were it not for trifling differences between the refrac- 

 tive index of the crown-glass quadrangle and that of the crown- 

 glass selected for the front of particular objectives. Measured 

 in this way it was easy to see experimentally that the limit of 82° 

 interior angle, which Mr. Wenham had assigned to all objectives, 

 did not practically exist. He himself, Professor R. Keith, and 

 lately Professor Stokes, of Cambridge, had shown that the rea- 

 soning on which Mr. Wenham based his assertion was not in 

 accordance with optical theory. Experimental observation indi- 

 cates that the number of makers who produce objectives with 

 more than 82'^ interior angle is increasing. This was the case in 

 this country, especially with many of the immersion objectives of 

 Tolles and Spencer; in England, with some of the immersion 

 objectives of POwell and Lealand, and on the Continent with the 

 immersion objectives of Zeiss. From the latter maker he had 

 recently received two of his new oil-immersion objectives, a ^th 

 and a -j-Mh. As measured by the glass quadrangle by sunlight, 

 the ^th'had an interior angle of 115°, the ^^^^th "of 114°. Mea- 

 sured by the apertometer of Abbe the results were, as nearly as 

 could be estimated, identical. Both objectives exceeded a little 

 the number 1.25 on Abbe's arbitrary scale, which corresponds to 

 113^ interior angle; but owing to the character of that scale the 

 exact amount of excess could only be estimated. Mr. Woodward 

 regarded the adoption of that scale as an unfortunate one. A 

 simple division of the glass circle into degrees would have been 

 in many respects more convenient. 



Mr. J. E. HiLGARD made a communication on 



JABLOKOFF'S ELECTRIC CANDLE. 



Ml'. W. n. Dall commenced a paper entitled 



NOTES ON THE MUSEUMS AND ZOOLOGICAL GARDENS OF NORTHERN 



EUROPE. 



