296 



THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



again applied two or three times in the manner before directed, the 

 dry pumice being powdered over the plate each time and polished 

 off gently with dry cotton. Care must be taken not to breathe upon 

 the plate or touch it with the fingers, for the slightest stain upon the 

 surface will be a defect in the drawing. It is indispensable that the 

 last operation with the acid be performed imme- 

 diately before it is intended for use. Let every 

 particle of dust be removed by cleaning all the 

 edges, and the back also, with cotton. After 

 the first polishing the plate C is fixed on a 

 board by means of four fillets, B B B B, of 

 plated copper. (Fig. 3.) To each of these are 

 8 soldered two small projecting pieces, wdiich 



hold the tablet near the corners; and the whole 

 is retained in a proper position by means of screws, as represented 

 at D D D D. 



Second operaf ion. —It is necessary for this operation, which is 



b3= 



really the most important of all, that a box similar to figures 4 and 5 

 oe provided. Figure 4 represents a section supposed to pass down 

 the middle of the apparatus by the line A B in figure 5, which repre- 

 sents the box as seen from above. C is a small lid which accurately 

 fits the interior and divides the boxes into two chambers. It is kept 

 constantly in its place when the box is not in use, the purpose of it 

 being to concentrate the vapor of the iodine that it may act more 

 readily ui)on the plate when it is exposed to it. D is the little cap- 

 sule in which tlie iodine is placed, which is covered with the ring J, 

 upon which is stretched a piece of fine gauze, by which the particles 

 of iodine are prevented from rising and staining the plate, while the 

 vapor, of course, passes freely through it. E is the l)oard with the 

 plate attached, which rests on the four smaller projecting pieces F, 



