66 PHOTO-MICROGRAPHS. 
be quite dark when this focusing is done, or light 
must be excluded from behind the screen by 
means of a black cloth thrown over the head, as 
in portrait photography. A very good plan is to 
have a shutter or curtain, by which all light is 
excluded from the window, covered with yellow 
paper. Or over the scantling which supports the 
camera a strip of cotton-velvet may be thrown in 
such a manner as to fall at the sides of the screen 
and exclude the light during the operation of 
focusing. 
It will be found that delicate details of difficult 
objects cannot be seen upon the ground-glass: for 
this reason, i 1s best to focus upon the aerial image in 
the position which will be occupied by the sensitive plate. 
A focusing-glass is required for this purpose, and a 
piece of plain glass is substituted for the ground- 
glass of the camera. 
As the ground-glass is also required for the 
rough focusing of the image from the microscope, 
it is well to have two swinging shutters, an exter- 
nal one containing the ground-glass, and an in- 
ternal one carrying the plain-glass plate. When 
the focusing is completed, these shutters are swung 
back to make place for the plate-holder. As ordi- 
nary cameras are not provided with two swinging 
shutters, the ground-glass may be held in a frame 
smaller than the frame which holds the plain 
glass, and this can be placed in position and re- 
tained by a button, or removed entirely, when not 
required. 
The light-proof connection between the screen 
