THE HISTORY OF SOME DISCOVERIES OF PIIOTOGR.VPHY. 297 



figiirb 5, G is the lid of the box, which is kept closed, except when 

 the plate is reinovcil or inserted. II represents the supports for the 

 cover C; K K, tapering sides all ronnd, forming a funnel-shaped box 

 within. 



To {)re})are the ])late: The cover C being taken out, the cup D i.s 

 charged with a suilicient quantity of iodine, broken into snudl pieces, 

 and covered with the gauze »I. The board E is now. with the plate 

 attached, placed face dowuAvard in its proj)er ])osition and the box 



carefully closed. 



In this position the plate remains until the vapor of the iodine has 

 produced a definite golden-yellow color — nothing more or less. If 

 the oi)eration is prolonged beyond the point at which this effect is 

 produced, a violet color is assumed, which is much less sensitive to 

 light, and if the yellow coating is too pale the picture jDroduced will 

 prove very faint in all its parts. The time for this can not be fixed, 

 as it depends entirely on the temperature of the surrounding air. Xo 

 artificial heat must be applied, unless in the case of elevating the 

 temperature of an apartment in which the operation may be going on. 

 It is also important that the temperature of the inside of the box 

 should be the same as it is without, as otherwise a dej^osition of 

 moisture is liable to take place over the surface of the plate. It is 

 well to leave a portion of the iodine always in the box, for, as it is 

 slowl}^ vaporized, it is absorbed by the wood, and when required it 

 is given out over the more extended surface-more equally and with 

 greater rapidity. 



As, according to the season of the year, the time for producing 

 the required effect may vary from five minutes to half an hour or 

 more, it is necessary, from time to time, to inspect the plate. This 

 is also necessary to see if the iodine is acting equally on every part 

 of the silver, as it sometimes happens that the color is sooner pro- 

 duced on one side than on the other, and the plate, wdien such is the 

 case, must be turned one quarter round. The plate must be inspected 

 in a darkened room, to which a faint light is admitted in some indi- 

 rect way, as by a door a little open. The board being lifted from the 

 box Avith both hands, the operator turning the plate tow^ard him 

 rapidly, observes the color. If too pale, it must be returned to the 

 box; but if it has assumed the violet color it is useless, and the whole 

 jDrocess must be again gone through. 



From description, this operation nmy api)ear very difficult, but 

 wdth a little practice the precise interval necessary to produce the 

 best effect is pretty easily guessed at. When the proper yellow^ color 

 is produced, the plate must be put into a frame, which fits the camera 

 obscura, and the doors are instantly closed upon it to prevent the 



