286 Transactions of the Boyal Microscopical Society. 
must approacli to a line, and cut off all rays in the focal plane on 
either side, quite up to the axis of the object-glass. To ensure this 
condition I now adopt the following method of measuring aper- 
tures : — a is the working diameter of an object-glass ; h, the central 
pencil, or true angle of aperture ; c, c, oblique or lateral pencils 
enclosing the field of view ; (i, 6? is a slit of considerable width, 
with parallel edges attached to a glass slip, e. In order to measure 
apertures, the object-glass is first adjusted and focussed on the 
upper surface of the glass slip. One edge of the slit is now 
brought forward so as exactly to bisect the field of view, half of 
which will appear quite dark. Over the eye-piece is now placed a 
cap containing a biconcave lens of about half an inch radii ; by 
means of this and the movement of the sliding containing tube, a 
distinct telescopic image of a distant lamp, or other bright object, 
may be obtained through the open half of the object-glass. Turn 
the open end away from the lamp by rotating the microscope, and 
the flame will suddenly disappear at the point when it is obscured 
by the edge of the slit. Mark this as zero ! Now remove the lens 
from over the eye-piece, bring back the slit till the opposite edge 
obscures the other half of the field, and again exactly bisects it, 
seeing that plane e is still in focus, replace the cap and turn the 
microscope, till the flame again vanishes, and the true aperture 
will be indicated. 
It will readily be seen by the diagram how the rays, c, c, of the 
obhque pencils, which have hitherto given a false indication in 
excess of aperture, are cut off by the edges. The thickness of 
these by this method is of no consequence, as it is the bottom of 
