HOW TO PHOTOGRAPH MICROSCOPIC OBJECTS. 
63 
lightproof as before. The index on the card (M) is placed well 
to the left of the dial, the focussing rod is attached, and all is 
ready for focussing the object. Much care and great accuracy 
will be required in this process, and a small handglass will be 
found very useful, taking care to look through it always in the 
same direction, for the focus on the screen varies materially as 
we regard it directly in a line with the light or obliquely. My 
own plan is to hold the lens opposite the centre of the focussing 
screen (a point marked on the collodion) at a distance of some two 
inches, then, when the point of light has been caught, to move the 
eye steadily backwards until the whole object is clearly shown. 
The focussing rod is now pushed forwards, so as to throw the ob- 
ject nearly out of focus, and gradually drawn back until the point 
of sharpest definition is reached. In this way no risk is run of 
■ focussing the distal side of the object. If the light is now evenly 
distributed over the object, it will be well to try a few flashes of 
the magnesium wire, to be sure that it is in the right position. 
About a quarter of an inch of the ribbon is allowed to protrude 
beyond the tin trough, and the flame of a small spirit-lamp 
applied until combustion takes place. If all is satisfactory, the 
index on the dial is moved on the required amount, say, ^ of a 
revolution, and all is ready for preparing the plate. This should 
be of stout “plate,” or flatted crown, well roughened at the edges 
with a corundum file, to secure adhesion of the film during the 
repeated washings it will have to undergo. One kind of col- 
lodion should be selected and adhered to, preference being given 
to such as will give some toughness of film. 
The bath is that in ordinary use. When sensitised, the 
plate is removed to the camera, and exposed to a number of 
flashes from the wire, varying with the intensity of the light 
on the screen and the density of the object. With a good light 
and transparent object one or two will suffice, whilst in certain 
cases, where oblique light is used, as many as forty or more will 
be found necessary to get a satisfactory impression. 
During the burning of the light, a green shade or green 
spectacles afford great relief to the eyes, which, if unprotected, 
are dazzled and blinded by the brilliancy of the rays. 
We have used many developers, all of which answer very 
well; perhaps the best is as follows: — 
Protosulphate of iron, a drachm and a half ; 
Powdered sugar, half an ounce ; 
Glacial acetic acid, two drachms and a half ; 
Distilled water, four ounces. 
But it must not be left any length of time on the plate after 
the image is out, or fogging will invariably make its appearance 
in the subsequent processes. When the picture is confined by the 
