Laboratory Notes 165 
for A , 295 ohms for B. This would indicate an ioniser resistance of 
2*5 ohms, and an arc resistance of 107 ohms, but in any case the 
made-up resistance should be finally adjusted with the help of an 
ammeter under working conditions. 
Much variation exists in the design of reflecting prisms and mirrors 
for use at the eyepiece, and several very unsatisfactory forms are on 
the market. The most useful pattern which the writer has seen is 
made by the Spencer Co., and consists of a right-angled prism 
attached to a rather long arm which is pivoted to a collar. This collar 
normally fits some way down the draw-tube of the microscope, and 
consequently makes it possible to employ such eyepieces as the 
Zeiss K. 12, which are of an awkward shape. Bausch and Lomb 
make a useful form which has the same advantage but also includes 
a Nicol prism analyser which can be swung out of the way when not 
needed. 
The most convenient and efficient surface for the examination of 
projected images appears to be a good quality Bristol board, though 
a plaster of Paris surface has been recommended. If precautions are 
taken to exclude all extraneous light, by darkening the room and 
shielding the illuminant, quite satisfactory microphotographs can be 
taken by substituting a photographic plate or bromide paper for the 
drawing surface. Black light-proof fabric, such as velveteen, mounted 
on a wire or wooden frame, is preferable to cardboard for shielding. 
This form of camera is recommended as the simplest and at the same 
time the cheapest available, having regard to efficient working. 
Under favourable conditions direct sunlight can be used, reflected 
by the substage mirror, but unless a heliostat is available, the mirror 
will require frequent alteration. 
To avoid the possibility of the outfit being accidentally thrown 
out of adjustment, the components should be securely fastened to a 
stout baseboard. 
Didsbury, Manchester. 
January 14, 1922. 
