270 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
illumination of the object is best attained by placing, between source and 
object, the object-glass out of an ordinary low-power eye-piece. For 
the same purpose the author also invariably uses a bull’s-eye. 
With regard to illumination by monochromatic light, the author 
despairs of the method by means of a prism, owing to the uneven 
illumination of the field. The difficulties of rotundity and colour are 
next touched upon. The latter may generally be overcome by the help 
of orthochromatic plates and screens, except in cases where the staining 
is faint or faded. 
In objects with very fine markings there is generally a tendency to 
a want of contrast in the result. Such difficulties are very hard to 
overcome, for, owing to the wide angular aperture required in these 
cases, the condenser cannot be dispensed with. With regard to increased 
magnification, the author’s experience is that it is a mistake to try to 
strain a lens to more than ten times its initial power. 
Focusing in Photomicrography. — M. P. Francotte communicates the 
following, which has been translated from his manuscript: — “In photo- 
graphing a microscopical preparation, two slides, one of ground glass 
and another of colourless polished glass, are used in order to focus the 
image. By the first of these slides only a rough focusing can be 
effected, but it is possible by its aid to appreciate the amount of light 
on the field of the Microscope and also to judge if the image is equally 
illuminated in all its parts. The second slide through the intervention 
of a lens allows of the exact focusing. But here the fact that the 
image is not visible to the naked eye is a source of great inconvenience ; 
for it is impossible to bring back the object to the centre of the plate if 
it has been displaced during the operations. The lens which has to be 
moved about at a certain distance from the image formed on the glass is 
the only guide. 
In place of this slide I have long made use of one of yellow, red, or 
slightly smoked glass (the last for photographing with sunlight). On 
a yellow glass the image can be seen by the naked eye, and all its 
details cau be perfectly distinguished without the aid of a lens. Thus 
an exact focusing can he effected without losing the advantages afforded 
by the ground glass; for with the naked eye any displacement of the 
image can be observed, and by means of the lens the exact focut-ing can be 
effected just as with the colourless glass. Such are the advantages as 
regards the focusing of the image. But the tinted glass is of great 
service when the electric light or sunlight is used. The difficulty in 
focusing in the latter case is well known ; the eyes are dazzled by the 
excessive light and it is sometimes necessary to forego the advantages 
presented by the polished glass and lens. This inconvenience is con- 
siderably modified by the coloured glass, and the focusing can be 
effected without trouble (with or without the lens) if the glass is tinted 
in proportion to the amount of light which falls on the plate. In our 
opinion all microscopical cameras should possess three slides, one of 
ground glass, another of glass tinted yellow or red or slightly smoked 
(like the glasses for moderating the light in the Abbe camera lucida), 
and lastly a third of colourless glass to be employed in special cases. 
With the tinted glass the results will be soon found to be far superior 
and the images much more detailed than before its use.” 
