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LABORATORY NOTES 
The Projection Microscope 
By H. J. DENHAM, M.A. 
he simple projection apparatus described by Professor Small 1 
J[ has undoubtedly several advantages over the usual form of 
Abbe drawing camera, particularly as regards the comfort of the 
worker, which becomes a matter of some importance when many 
drawings have to be made; at the same time, it has its limitations 
as regards critical resolution, and is not to be recommended for cyto- 
logical or other preparations where differentiation in depth is im¬ 
portant. A similar form is very largely used in American laboratories, 
simple outfits having been placed on the market some time ago by 
two large optical firms there, but the illuminant is usually a gas- 
filled concentrated-filament lamp, which does not compare with the 
Pointolite as a source of light: while on the continent the Leitz small 
“Edinger” outfit is used, in which the source of light is a small arc, 
and the microscope is inclined at an angle of 45 0 , which has advan¬ 
tages over the horizontal position when temporary preparations are 
examined. 
There are two small improvements which might be made in Pro¬ 
fessor Small’s apparatus. The addition of a condenser to parallelise 
the rays from the illuminant is essential if a critical image is desired, 
and makes it possible to place the Pointolite at some distance from 
the microscope, simplifying adjustments and allowing more room 
for light filters and cooling troughs when required. The 100 c.p. lamp 
emits considerable energy in the form of heat, and the writer finds 
that the 30 c.p. pointolite is much more useful in the botanical 
laboratory. The bulb is much smaller, allowing a shorter focus lens 
to be used, placed much closer to the incandescent bead, with conse¬ 
quent increase in the intensity of illumination. 
The second improvement, to obviate the necessity of a night- 
light, is the insertion of a variable resistance in the arc circuit of the 
Pointolite as shown in the figure. This has been described by the 
1 “Notes on an easy method of camera lucida drawing,” New Phytol. 
20 , 1921, pp. 238-241. 
