116 THE BOOK OF THE LANTERN. 



the exposure of a chloride plate to lamplight will occupy 

 about three minutes, a bromide plate can be successfully 

 exposed in less than three seconds, so that a number can 

 be exposed in an hour or two. 



I have already pointed out how a reduced positive can 

 be readily obtained from a negative by means of the 

 camera. I will now suppose that the negative from which 

 the transparency is to be taken, is on a plate, and, there- 

 fore, the right size for the lantern slide, and that the pic- 

 ture is to be printed by contact in a printing frame. 

 The requirements are a red lamp, a gas or paraffin 

 lamp, which can be readily turned up and down, and a 

 frame. 



As in most photographic operations, correct exposure is 

 the main consideration, but whereas where daylight is 

 concerned, this exposure is always difficult to hit upon, 

 because the light varies so much under different circum- 

 stances ', here, where we have a lamp to work by, we can 

 measure its duration to a nicety. 



I may mention here that there is a form of gas-burners 

 sold which is very useful in this work. It is fitted with a 

 bypass, so that it is never actually turned out. A blue 

 bead of light remains, which is quite shielded from view, 

 and this permanent flame ignites the full amount of gas 

 when the stop-cock is turned on. The operation of print- 

 ing a transparency consists in placing the negative in the 

 frame, and placing upon it, film to film, a gelatine plate, 

 measuring 3 x 3 inches. The frame is closed, and is 

 held within a short distance of the lamp, which is turned 



