106 PHOTO-MICROGRAPHS. 
ance ot blood-corpuscles due to disease, if any 
constant peculiarities of this kind were associated 
with particular diseases. But the microscope does 
not reveal any such peculiarities of a sufficiently 
definite character to justify the expectation, at 
one time extensively entertained, that its use, in 
the examination of the vital fluid, might prove of 
value in deciding questions of diagnosis. 
Differences in the relative proportion of the 
white and red corpuscles are, however, shown in 
a rough way, and the depth of color of the red 
corpuscles is indicated, to a certain extent, by the 
photographic contrast with the ground; or, better 
still, with white corpuscles in the same field. 
The presence of foreign elements — parasitic 
organisms — is shown very satisfactorily in photo- 
micrographs; and if a sufficient power is used, 
their absence is rendered apparent when there 
are none. 
The method is therefore especially useful for 
recording facts of this kind, as the observer is able 
to substantiate the truth of his statements, positive 
or negative, by unimpeachable evidence, and at the 
same time to show that his skill as a microscopist 
is sufficient to give confidence in his ability to 
manipulate the high powers with which such 
observations are necessarily made. 
For example, the photo-micrograph of yellow- 
fever blood in the present volume, Plate VI., in 
which the amplification is nearly fifteen hundred 
diameters, and in which the white and red blood- 
corpuscles are well defined, may be taken as 
