ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
113 
diameter which is closed on both sides by round cover-glasses 21 mm. 
in diameter. The latter are let into the plate so that their upper sur- 
faces are exactly in the same plane as the surfaces of the plate. The 
two borings made in the side of the plate at a and b communicate, as 
seen in the figure, with the cylin- 
drical space enclosed between 
the cover-glasses. The object- 
holder is fastened to the cooler 
by means of the two clamps /, 
and the whole apparatus is set 
in a movable object-slide. The 
cooling is effected by a stream 
of cold water which enters by 
the pipe at a , and is carried off 
at b. Experiments made as before 
to determine the effect of the 
cooler showed that, when both 
coolers were used, in five minutes 
the temperature only rose 1°*5, 
from 25° to 26° *5, and then re- 
mained constant at 26° to 27°. In these experiments the thermometer 
was mounted in a mercury trough, 5 mm. thick, the sides of which 
were thickly coated with lamp-black. 
The cooler has the disadvantage that it limits the use of the 
ordinary Abbe condenser since it is impossible, owing to the thickness 
of the cooler (6*25 mm.), to adjust the focus in parallel light on the' 
plane of the object or near it. A special condenser, however, of greater 
focal length can be used, since for i^rojection purposes an aperture higher 
than 1*0 is not required. 
Orthochromatism applied to Photomicrography.* — Before the 
Societe Eran^aise de Photographie on May 5th, M. Monpillard read a 
paper on this subject, in the course of which he said that the design of 
photomicrography is, given an object visible to the naked eye, to obtain 
an enlarged image of it. scrupulously accurate both in form and detail. 
Nothing is more easy in the case of colourless objects, such as diatoms, 
sections of bones, &c., the great perfection of modern objectives per- 
mitting of the obtainment of great sharpness and detail of image. It is 
not equally so when the objects are coloured, either naturally or arti- 
ficially, and it may be said— for experience has proved it to be so — - 
that all coloration degrades the perfection of the photographic image, 
obtained with ordinary gelatin plates, so that if the object be of blue 
or violet colour (the most actinic colours), it stands out badly from the 
ground, and has its details poorly rendered. On the other hand, if it 
is yellow, orange, or r< d, the object is vigorously rendered on the ground, 
but the details, bathed, as it were, in a light that is powerless to affect 
the sensitive salt, will, if the object be green, be very small, and less 
so with yellow and red. 
In all cases, the employment of orthochromatic plates requires also 
the use of coloured screens. 
* Anthony’s Photographic Bulletin, xxiv. (1893) pp. 608-11. 
Pig. 10. 
1894 
l 
