PHOTOMICROGRAPHY. 119 
monochromatic light. ‘This can be obtained by causing the sun’s 
rays to pass through a solution of sulphate of copper to which is 
added strong ammonia. 
In photographing the delicate markings of some diatoms the 
light must impinge on these transparent objects at a considerable 
angle, or the direct flood of rays will drown all detail. In photo- 
graphing the proboscis of a blow-fly I found a superabundance of 
light fatal to the fine delineation of the false zvachea. With objects 
difficult of resolution the ordinary achromatic condenser of the 
microscope is often employed. 
Definition.—In the lower powers definition is rendered much 
more perfect by the introduction of a stop behind the back lens. 
For instance: in portraying a section of the stem of the dog rose, 
a stop a quarter of an inch in diameter materially improved the 
definition of the delicate cells, and in many histological specimens, 
where great penetration is required, the tissues being compara- 
tively thick, a reduction of the aperture of the lens is imperative. 
Stops of cardboard or turned wood may be employed, or Davis’s 
iris aperture shutter, which gives all variations in size from a pin 
point upwards. 
Odjects.—All microscopical objects are not equally suitable, on 
account of either colour or thickness. ‘Tissues stained light blue 
or purple give faint images, while dense brown objects will not 
allow light to penetrate the detail. Preparations of insects (such 
as fleas, which make capital subjects for the beginner) should have 
lain in the potash solution or turpentine a sufficient time to render 
the body semi-transparent. Sections of woods, if cut thin and 
stained a suitable colour, make good pictures. Sections of lung, 
if thin, give good results, but many anatomical preparations of soft 
tissues are too thick to allow of perfect focussing with the higher 
powers. A beautiful section of the retina of the human eye which 
I possess, although most interesting when viewed in the ordinary 
microscope, is quite unfit for the camera. Diatoms which present 
a flat surface, like arachnoidiscus, are most suitable, but require 
careful levelling on the stage. 
_ L£xposure.—No fixed rule can be given for exposure, which varies 
with the light, lenses, and length of focus employed, but much 
trouble will be avoided by the operator if a standard light be used, 
and a careful record kept of the results of exposures by means of a 
register, such as that which lies on the table. With Swan’s “ ten 
times” dry plates a small microscopic paraffine lamp without con- 
denser, fixed seven inches from the object, give a good picture in 
one minute, with a two-inch lens full aperture, and “the bellows €X- 
tended to thirty inches. 
As a rule, amateurs over-expose their plates, and produce a weak, 
thin negative. The movable disc before referred to affords an 
