54 American Quarterly Microscopical Journal. 



does, and I give this ingenious plan from a private letter 

 which I received from him a long time since. 



Harden a little balsam on a slide until it will retain any 

 form given to it, and, at the same time, yield to moderate 

 pressure. Place upon it a thin cover-glass. The total thick- 

 ness of balsam and cover must be greater than the focal 

 length of the objective. Lower the latter until the surface of 

 the slide is visible through the balsam and cover. By means 

 of a lever of contact, now measure the thickness of the bal- 

 sam and cover, which will give the desired focal length. 



With the cover-glass and face of the objective in contact, as 

 in this method of Wenham, the glass acts practically as an 

 immersion, and, unless vision is distinct., the angle so measured 

 does not indicate any angle of aperture proper to the objec- 

 tive. 



TO MEASURE THE AVAILABLE FACE OF THE LENS. 



Place the objective on the table, face up. Put a drop of 

 milk upon the front lens, and when dry place the objective in 

 the sub-stage, as for a condenser ; throw parallel rays up 

 through it, and, by means of a low power objective and eye- 

 piece micrometer, measure the diameter of the spot of light 

 seen when the dried milk surface is in focus. 



We have now simply to calculate the angle by any mathe- 

 matical formula, the base and perpendicular of the triangle 

 having been thus determined. 



I propose now to enumerate the results we may expect from 

 an universal adoption of this method. It would place within 

 our hands a means of confining future discussion within legit- 

 imate bounds. The term angular aperture would mean some- 

 thing definite, and there would be no excuse for any misappre- 

 hension on any side. 



Look, however, at the field of research which it points out — 

 thus far practically disregarded. From its nature angular 

 aperture will mean, for a dry objective, the angle measured in 

 air ; for an immersion, the angle measured in the fluid used. 

 At once the question arises, what is the angle of light which 

 comes from an object in balsam or other medium ? The angle 

 within the medium being different from the angle at which 

 the light enters the objective, which of these angles has the 

 most to do with the resolving power ? What relation is there 



