8 
DESCRIPTION OF THE APPARATUS. 
Success in microphotograpliic work depends largely upon tke 
illuminant, of wliick there are six for consideration: Sunlight, 
the electric arc light, the oxyhydrogen light, illuminating gas, 
coal oil, and acetylene. Sunlight gives excellent photographs but 
is excluded on account of the expensive apparatus and special 
requirements for good work. The electric arc gives excellent 
results and is necessary when magnifications above 1,500 diameters 
are desired. The oxyhydrogen light, illuminating gas, and coal 
oil are too feeble for any but low diameters. Acetylene, because 
of its brilliancy, high actinic properties, low heat reflection, port- 
ability, low cost, and adaptability to other laboratory work, is the 
illuminant par excellence for diameters up to and including 1,500, 
for low-power microscopic projection, and lantern-slide projection. 
The illuminating apparatus in use at this laboratory consists of 
a generator large enough to furnish gas for a quadruple burner, 
two condensers, of 4-inch focus, and a light filter and an absorbing 
cell holding a layer of water ^ inch thick. The latter is suffi- 
ciently large to remove all heat rays as well as serving to hold 
water tinged with a color complementary to the stain employed on 
the specimen to be photographed. 
The bellows as described above is one of the numerous models 
for lantern-slide work, either 3d by 4d or 4 by 5 inches. 
The whole apparatus is mounted upon a firm base. 
FOCUSING. 
One of the most difficult features in connection with micro- 
photographic work is focusing the object on the. ground-glass 
screen. We have discarded the various focusing rods used for this 
purpose and now depend upon a braided twine, the center going 
around the fine adjustment and thence to the back of the camera, 
where it is kept taut by small weights. This has the advantage 
that when the focus is accurately found the ends can be released, 
removing the strain from the fine adjustment and consequently all 
interference with the focus. Rods clamped to the fine adjustment 
are apt to change the focus when released and are acted upon by 
vibrations of the room. 
A magnifying glass of low magnification can be used as an aid 
in focusing with high diameters, but as a rule the focus of the 
entire field can be much more accurately gauged with the eye 
alone than with the aid of lenses. 
EXPOSURE. 
The length of exposure will depend upon the intensity of the 
image upon the screen and upon the rapidity of the plate. A 
