10 THE AMERICAN MONTHLY [ January, 
same condition as when observing with an eye-piece. After a preliminary 
adjustment of the specimen by means of the ordinary eye-piece, the projection 
eye-piece is put in its place and its projection lens so adjusted that the edge 
of the diaphragm is focused as sharply as possible on the screen or ground 
glass of the photographic camera. This is accomplished by drawing out 
the projection lens more or less, according as the distance between the screen 
or plate and the microscope is reduced or increased. Finally, the image of 
the object is sharply focused on the screen or ground glass by the usual adjust- 
ments. The length of body for which the objective is adjusted for observa- 
tion with an eye-piece must always be exactly retained. 
The cap of the projection eye-piece forms a diaphragm, by which any false 
light from the body-tube is completely shut off. The size of the aperture of 
this diaphragm corresponds with the highest aperture of the apochromatics. 
When using either those of 0.6 or 0.3, it may occasionally be desirable to 
decrease the available aperture of the objective in order to obtain uniform 
sharpness of definition up to the margin of the field. For this purpose each 
projection eye-piece is supplied with two diaphragms of smaller apertures, 
which fit in place of the normal one. It must not be forgotten to remove 
these from the eye-piece if the full aperture of the objective is to be effective. 
Projection by this method gives extremely sharp uniformly illuminated 
pictures of any desired degree of magnification. 
The projection eye-pieces are speedily corrected for the apochromatics, on 
the principle of the compensating series of eye-pieces, but may, nevertheless, 
be advantageously employed with ordinary achromatic objectives of large 
aperture. They are constructed for both Continental and English micro- 
scopes on somewhat different formule, according to the difference in tube- 
length. There are two numbers for each series, giving an eye-piece magnifi- 
(2 and 4 for 160 mm. body. 
egrane. oUF 250 7" a 
compensating eye-pieces, the ratio in which, by means of the eye-piece and 
the given length of body-tube for which it is adjusted, the focal length of the 
whole microscope is less than that of the objective alone (in so far as the 
eye-piece is adjusted to a great distance). 
For instance, the projection eye-piece 2 diminishes the focal length of each 
objective by exactly one-half; an objective of 3 mm. therefore will, with 
this eye-piece, project as large an image as an objective of 1.5 mm. without 
it, the screen or plate remaining at the same distance. 
As the linear magnification of a projected image is the quotient obtained by 
dividing the distance of the image from the posterior focal point of the objec- 
tive by the equivalent focal length of the latter, we can determine the magnifica- 
tion at any distance of the image from the eye-piece by dividing this distance, 
expressed in mm., by the focal length of the objective used, and multiplying the 
result by the number of the projection eye-piece employed. Thus the objec- 
tive of 3 mm. givés, with the projection eye-piece 2, an image magnified 1000 
cation of These figures indicate, as in the 
: : A 1500 : 
times at a distance of 150 cm. (2 - G2 100 ). This rule holds good for 
greater distances, but in the case of smaller it gives too high a reading. 
The diameter of the image, or the screen or plate, when the eye-pieces 2 
and 3 are used, is about + of the distance of the image, and with 4 and 6 about 
4 of that distance. 
The image distance may be reduced in the case of 2 and 3 to about 400 
mm., and in 4 and 6 to about 250 mm., reckoning from the eye-piece. It 
can be increased to any desired amount. 
For purposes of demonstration and for photo-micrography, where only 
small pictures are required, or in cases where the plate can be placed at a 
27m 
