POSITIVES UPON GLASS. 89 
XIII. POSITIVES UPON GLASS. 
PosITIVEs on glass are used mainly for lantern 
projections. They are admirably adapted for the 
illustration of popular lectures, and are made with 
such facility that one who has a good series of 
negatives can always produce them at short 
notice. No camera is required, as the process 
by contact gives excellent results, especially with 
dry-plates. 
The writer’s method of manipulation is as fol- 
lows: The negative is placed in the plate-holder 
with the film side to the back; the dry-plate is 
placed in contact with this film to the front; a 
piece of black cardboard of the same size is placed 
back of this; the plate-holder is then closed and 
held up in the proper position with reference to 
the light; the shutter is drawn quickly, to make 
the exposure, and quickly replaced after the proper 
interval, which will always be very brief. 
A steady artificial light— gas or oil— may be 
used ; but the writer prefers to use daylight, admit- 
ted to his operating-room through a small opening 
in one of the windows. The size of this opening 
is regulated by means of a sliding shutter, cov- 
ering a single pane of glass when closed. A 
slit an inch or two wide will admit an abundance 
of light, when the exposure is made on the oppo- 
site side of a room ten or twelve feet wide. 
Even with this narrow opening, the time of ex- 
