394 
Transactions of the Society. 
the illumination per unit area. In this case, therefore, if the 
illumination is sufficient without a bull’s-eye to enable an object 
to be seen clearly with the naked eye, the use of a bull’s-eye in 
the manner here described allows the object to be equally well 
seen with a microscope magnifying about 7 diameters. 
Secondly, when the bull’s-eye is at a distance of 300 mm. from 
the source of light and 100 mm. from the object, the same formula 
gives 
KL' 2 _ K *2642 _ J 
a/ 2 10,000 3-8* 
Here the brilliancy of the object is increased from to 
about 420 times, and will allow of the use of a microscope 
magnifying about 20 diameters. 
A substage condenser of a focal length of 8 mm. used without 
a bull’s-eye produces a sharp image at a distance of 8*16 mm. 
from the object. If the source of light be as before at a distance 
of 400 mm. from the object, the substage condenser acts exactly as 
a bull’s-eye, and the illumination becomes 
KL' 2 _ KL' 2 
x' 2 (8*16) 2 ’ 
We are, however, not illuminating an opaque object scattering 
light in all directions. We are passing light through a transparent 
object, and use a microscope object glass which can only receive 
light of a definite angle from the object. Suppose an object glass 
to have a focal length of 8 mm. which can only receive light at an 
angle of 0 * 5 1ST. A. or 60°. The substage condenser may be of a 
larger size than is required to admit 60°, but if so the light that 
is thrown through the object at a greater angle than this will not 
be collected by the object glass, and need not be considered ; there- 
fore, for the purpose of the investigation the substage condenser 
may be considered to have a diameter of 8 mm. and an area of 
50*26 mm. The light received by the object that will enter the 
object glass will be 
K 50-26 K 
(8-16) 2 1*3 
an increase of about 1230. 
How interpose a bull’s-eye 75 mm. focal length 58 mm. at 
75 mm. from the light. 
The bull’s-eye will direct beams of parallel light from each 
point of the source of light upon the substage condenser which 
