397 
The Photometry of a Bull's-Eye Lens. 
where the refraction is almost the same as the cover glass and 
slip, are not increased in brilliancy when a bull’s-eye is used, and 
that such increase must be due to reflection from the cover glass 
and slip. 
That this may be so may be illustrated by the following 
diagrams. 
If, as in fig. 4, the image formed by the illuminator of the 
source of light is very small, such light as is reflected by the 
came gLajs 
cover glass does not come back to the object and increase its 
brilliancy ; but if, as in fig. 5, the image formed by the illuminator is 
large, then all light from distances a, b and c from the centre will 
be partially reflected by the cover glass in such a manner that they 
fall upon the object in the centre. The effect of the bull’s-eye is 
to produce a large instead of a small image of the source of light 
d c (> ct al> c <1 
anwv gflass 
Fig. 5. 
on the object, and it would appear that this may be the reason 
for the increase in light observed in the case of a bull’s-eye 
on such specimens as living bacteria in water. The film is very 
thin, and many sets of reflections as indicated in fig. 5 might take 
place. 
If an object such as paper, or even a section of tissue, is of 
itself a diffusing or reflecting material, each particle that is illumi- 
nated reflects light in all directions, and it is easy to understand 
