
THE NORTHERN MICROSCOPIST, 
AND 
MicroscoricaL News. 

No. 24. DECEMBER. 1882. 




THE THEORY OF APERTURE IN THE 
MICROSCOPE: A POPULAR EXPOSITION.* 
By W. Biackpurn, F.R.MLS. 
“ 4 PERTURE” has been truly described as the capacity of the 
object-glass for receiving light from the object and transmit- 
ting it to the magnified image. If we admit this to be a correct 
definition’ of the function of aperture, and to me it appears to 
be a self-evident proposition, then there only remains for us to 
investigate the conditions under which light is so received and 
transmitted. 
Until within the last few years, it was the custom to estimate the 
aperture of an objective by the angular extension of a cone of light 
impinging upon the front surface, and having its apex or radiant- 
point in the object, or as much of this cone as was transmitted by 
the lens. (See fig. 34.) Fig. 39. 
The angle of trans- 
mitted light was mea- 
sured by placing the 
body of the microscope 
in a horizontal position 
in front of a lamp ten 
or fifteen feet distant, 
and, upon a rotating 
circular platform with a 
graduated edge, moyv- 
ing the microscope to 
each side until the edge 
of the flame appeared 
to divide the field in \’ 
the eye-piece; and the FALSE THEORY. TRUE THEORY. 
angle of rotation, read Fig. 36. 
upon the graduated arc, indicated the angular aperture of the ob- 


* A paper read before the Manchester Microscopical Society on Nov. 2nd. 
