CHAP. VI.] 



PHOTOGRAPHY. 



293 



light of a candle whether the desired result was obtained ; the silver 

 coating should then have taken a beautiful golden hue. This opera- 

 tion required from three minutes to half an hour, according to the 

 temperature. 



The plate thus prepared was then placed at the focus of a lens in 

 the camera, care being taken not to leave a longer interval than an hour 

 between this third operation and the preceding one. 1 The objects to 

 be copied were placed in the direct light of the sun.' After this plate 

 had been exposed for a certain time, varying with the time of day 

 and with the season and which for Paris was three minutes at least 

 and thirty minutes at most the 

 photogenic action of the light was 

 complete. The plate, on which 

 nothing was yet visible, and from 

 which the light had to be care- 

 fully excluded, bore a faithful 

 impression of all the objects which 

 had co-operated in sending to it 

 luminous rays. 



It only remained then to de- 

 velop this image, hidden as it 

 were beneath a veil, and to hx it 

 so as to preserve it from fading. 

 Daguerre thus proceeded to effect 

 these last two operations. 



The plate was put inside a box, 

 and the impressed surface, inclined 



at an angle of 45, was submitted to the action of vapours, which 

 escaped from a capsule containing mercury heated to a temperature of 

 60 to 75 centigrade. After some minutes the picture began to appear 

 and to become more and more clear and accurate, a result which would 

 be watched by the light of a candle. After the temperature of the 

 mercury was lowered to about 45 the operation was complete, and 

 the proof perfect. It could then be kept without changing for several 

 months, if it were not exposed often to daylight. " The object of the 

 fifth operation," says Daguerre, " is to clear the plate from the iodine, 



1 It was subsequently found that the sensitiveness of the plate was increased by 

 allowing the iodized plate to remain half a day before exposure. 



FIG. 217. Mercury box lor developing 

 daguerreotypes. 



