84 PHOTO-MICROGRAPHS. 
the operating-room, and to place a line of nails, in 
pairs, along the centre of this. These nails should 
be about two or three inches apart, and are only 
partially driven home. The negative is supported 
between them in an oblique direction, only the 
lower corner touching the board. 
Negatives may be defective from any of the 
following named causes : — 
Improper adjustment of the illuminating apparatus, 
causing the plate to be unevenly lighted; light 
from heliostat or achromatic condenser not well 
centred; or obscuring details from imperfect 
focusing of condenser. 
Foggy appearance, due to the admission of ex- 
traneous light to the plate, —from beneath the 
object, to be remedied by diaphragms; from dif- 
fused light, or light reflected from the stage en- 
tering the objective, remecied by working in a 
dark room; from cracks or imperfect connections 
in the plate-holder and camera. 
Circle or spot of light, due to reflection from 
interior of tube of microscope, to be remedied by 
cylinder of black paper; or from reflecting 
surface upon interior of camera or its connections 
with the microscope — remedied by blackening. 
Imperfect definition, from use of inferior lenses ; 
imperfect cover-correction; straining the lens, as 
to distance from screen, beyond what it will stand ; 
improper adjustment of amplifier, if one is used, 
in the tube of the microscope; or imperfections 
introduced by eye-piece, if the attempt is made to 
use one. 
