MICROSCOPIC POWER OF OBJECTIVES. 
227 
high a power as the l-25tli inch is readily and safely brought 
down into focus by the coarse adjustment alone, leaving the 
line adjustment to be used only in the further examination of the 
object; and definite notice is received at once if the cover- 
glass of the object happens to be too thick for allowing the 
focussing to he effected, the only care that is requisite being 
never to go so far in the focussing movement as to stop all 
freedom of motion in the loose stage plate. 
In the illumination of the object, the light is concentrated 
as usual by an achromatic condenser, so as to get a sufficient 
quantity of light passed through the minute face aperture of 
the objective, and a reflecting prism is used instead of the 
ordinary mirror, because with the mirror there are two 
reflections of every ray—the principal reflection from the 
silvered back of the glass, and a secondary one from the face 
of the glass,—and the mixture of these two sets of rays 
interferes with distinctness of definition. Special care has 
to be taken in the adjustment of the illumination, to get the 
direction of the light truly in the axis of the microscope and 
central in position, for obtaining a sharp definition with the 
objective. The immersion objective has an advantage in 
illumination over a dry objective, because the intervening 
film of water that is in contact with both the cover-glass and 
the face lens prevents the loss of light that occurs from 
partial reflection of the oblique rays by the exterior surfaces 
of those two glasses that occurs in a dry objective, on account 
of those surfaces being exposed to the air. 
An object now shown in the microscope, under the l-25tli 
inch objective, is a minute circular diatom, Aulacodiscus 
formosus , which is only 1-lOOth inch diameter (as small as 
the finest needle), and this is magnified to more than 3 feet 
diameter with the highest power, only l-8th part of the 
diameter being then visible at once. The surface of this 
diatom is beautifully marked with rows of dots symmetrically 
arranged, at a pitch of about 80 to the diameter of the 
diatom (1-lOOth inch), and the dots are consequently about 
1-8000th inch apart; these are magnified to nearly J inch 
apart, making a magnifying power of nearly 4000 diameters. 
This power is measured by drawing the magnified object 
direct from the microscope with a camera prism, the distance 
of the paper upon which the image is drawn being fixed 
at 10 inches from the aperture of the eye-piece, which is 
the standard distance used for measuring the magnifying 
power of microscopes. A transparent micrometer scale 
divided into 1-lOOtlis and l-1000ths of an inch is then laid 
upon the microscope stage in the place of the object, and 
