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IV. — The Slit as an Aid in Measuring Angular Aperture. 
By Professor K. Keith. 
It has recently been proposed to use a very narrow slit, on the 
stage of the microscope, to prevent the supposed effect of stray 
light in measuring the aperture of objectives. 
This device could hardly mislead anyone who keeps in mind 
the nature of spherical aberration. Most observers with the micro- 
scope, however, give but little attention to the abstract principles of 
the instrument, and therefore a graphic representation of the effect 
of aberration will probably be of interest. 
I present two figures, which correctly show the direction of the 
light in the two cases specified : all the angles being calculated and 
measured. 
The objective is adjusted in both cases so as to be free from 
aberration when the under surface of the glass cover is in focus, and 
the upper surface touches the objective. The lines marked 40°, 
38°, &c., represent the direction of the rays of light, the figures in- 
dicating the angles that they make with the axis, and a h represents 
the face of the objective. 
Pm. l. 
In the first figure, with the glass cover interposed, rays from 
a small luminous object near the eye end of the tube, will all be 
collected into a very small image at the point g. In this case there 
is no aberration, and no stray light, and any of the devices for mea- 
