62 PHOTO-MICROGRAPHS. 
by such an arrangement as has been described. 
Light comes from all directions, and is reflected 
on the object from the white marble slab of the 
working-table as well as from the mirror. The 
polished stage of the microscope, especially with 
low powers, also reflects rays into the objective, 
and diffused or reflected light enters the eye-piece, 
and also the pupil of the observer’s eye, to add to 
the confusion of the image. It is for this reason 
that many persons prefer to work by lamplight. 
The source of light being under control, these 
difficulties are avoided, with a decided gain of 
definition. But the quality of the light reflected 
from the sky is unsurpassed; and not only is the 
best definition possible, with the optical apparatus 
employed, obtained by this means, but the light is 
more agreeable and less injurious to the eye of 
the microscopist than is lamplight. 
When a heliostat is used, the brief time of ex- 
posure makes it a simple matter to exclude extra- 
neous light from the sensitized plate, and the dark 
room itself serves perfectly well as a camera. But 
without a heliostat, when the exposure is some- 
times ten minutes or more, an extremely small 
amount of diffused actinic hght in the room is 
sufficient to ruin the very sensitive dry plate. The 
operator would find it irksome to remain a pris- 
oner in the dark room during these long expo- 
sures, but he cannot safely open a door for the pur- 
pose of leaving it, nor can he make use of a lamp 
for the purpose of reading, or of attending to other 
matters. For this reason it is desirable to have 
