MEASURING UPON THE SCREEN. 67 
and the microscope must be arranged before the 
final focusing, as any disturbance of the apparatus 
is likely to disarrange the adjustment. 
VII. MEASURING UPON THE SCREEN. 
THE facility with which microscopic objects can 
be measured when projected upon a screen, in the 
manner described, or from the photographic im- 
print of the image, is one of the advantages of this 
method. 
To ascertain the real dimensions of the object 
from the measurements made of the enlarged 
image, it is, of course, necessary to know the exact 
amplification. This is ascertained by projecting 
upon the screen the lines ruled upon a stage- 
micrometer. If great accuracy is desired, the 
operator will do well to obtain one of the stage- 
micrometer slides made by Professor W. A. Rogers, 
of Cambridge, Mass. 
Micrometer slides are ruled in fractions of an 
inch or of a centimetre. The usual unit of measure- 
ment for the smallest microscopic objects is the 
micromillimetre, for which the Greek letter p 
serves as a symbol. The micromillimetre is the 
one-thousandth part of a millimetre —z.e. 1,000 
p= 1 mm.—, which is equal to the one-twenty- 
five-thousandth part of an inch. The English mi- 
crometers are ruled in one hundredths and one 
thousandths of an inch. 
If we project upon the screen the lines on a 
