70 PHOTO-MICROGRAPHS. 
tions must be identical as regards the position 
of the optical appliances, or a certain amount of 
error will result. Thus, if the amplifier is moved 
to a different position in the tube, or the cover- 
correction of an objective is changed, the magni- 
fying power of the combination will be slightly 
altered. 
VUI. DIAPHRAGMS IN THE PLATE-HOLDER. 
A DIAPHRAGM with a circular or oval aperture 
introduced into the plate-holder, in front of the 
dry plate, is a convenient way of giving a good 
form and definite boundary to the field. The 
light is arrested by this diaphragm, so that the 
part covered is transparent in the negative, while 
in the positive the picture appears framed in black. 
(See Plate XI. Fig. 2.) The effect obtained in 
this way is especially good when the negative is 
to be used for printing positives on glass for 
projection. 
Silk thread or horsehair may be stretched 
before the plate for the purpose of printing fine 
black lines upon the positives. The writer some- 
times resorts to this device for the purpose of 
showing the amplification upon the screen when 
the glass positives are to be used to illustrate 
popular lectures. Black cardboard may be used 
for diaphragms in front of the plate, and lines may 
be drawn by attaching black thread or horsehair 
to this with sealing-wax. 
