PHOTO-MICROGRAPHIC APPARATUS. 
5 1 
carefully lighted on a piece of white card which is in 
front of the ground-glass, and is viewed by raising the 
sliding door seen at the side of the camera. 
For this kind of work the “ Planar ” lenses of Zeiss 
are singularly well adapted, but any well-corrected, 
Hat-fielded lens will answer. The Planars of 35 and 
70 millimetres respectively have proved themselves 
specially convenient in our work, and they need not, 
in fact must not, be much stopped down. The “Holo- 
scopics ” of Watson and Sons also act admirably. 
The beginner will find his chief trouble with the 
lighting; great care must be taken to get this per¬ 
fectly even as seen on the white card. The flame 
must, of course, be at or near the principal focus of the 
condenser, and the condenser should be pretty close to 
the object, but a good deal of tentative moving of the 
parts will probably be required to get the illumination 
quite even. Especially with the shorter-focus lenses of 
the Planar type the aperture must be kept fairly wide, 
as otherwise the “ depth of focus ” of the lens gives 
rise to an image of the light more or less sharp, which 
at once spoils the lighting. 
The easiest way to secure even lighting is to put a 
piece of very finely ground-glass between the condenser 
and the stage, and even here care must be taken not to 
get an image of the grain of the glass. A very good 
way to obtain even illumination is to use a second lens 
of suitable size and “ figure ” between the paralleliser 
and the stage. 
With this apparatus, after the fine adjustment was 
added to it, the writer has worked quite successfully 
