140 The Daguerreotype and its Applications. 



It has been stated that the method recommended by M. Da- 

 guerre for procuring a pure silver surface, might be abridged, and 

 some of the steps omitted. Any omission, however, in this re- 

 spect, is attended with risk to the success of the operation. A 

 new plate must be ground to an even surface, polished, burnt, re- 

 polished, washed with dilute nitric acid, rubbed with some dry 

 and fine powder, and finished with dry cotton. Of the polishing 

 powders usually employed, fine emery is the best to bring the 

 plate to an even surface, and to remove the deep scratches, tripoli 

 for polishing, and whiting to succeed the dilute acid. 



In every stage, the rubbing of the plate should be performed 

 transversely, and in the direction opposite to that in which it is to 

 be held to view the picture. If the longer diameter of the plate 

 is to be vertical, the polishing strokes should be parallel to the 

 shorter diameter ; and the reverse if the shorter diameter is to be 

 vertical. 



A re-application of the dilute nitric acid, and of the whiting, 

 is necessary if the plate is kept an hour or two before it is iodized. 

 It should also, after having been exposed to the mercurial vapor, 

 be re-polished and burnt before it is again employed, to evaporate 

 any mercury that may adhere to its surface, but more particularly, 

 to prevent the reappearance of the first proof along with the second. 



If the iodization has been carried beyond the golden yellow, 

 the coat is less sensitive ; the proof is also liable to be stained by 

 the light which is reflected from the plate itself, to the lens, and 

 to the sides of the camera, and which is reflected back again in- 

 discriminately over the iodized surface. Light of this color ap- 

 pears, after suffering these two reflections, to exert no influence 

 on the sensitive coat in the period required to take a proof. Proofs 

 can be obtained, if the iodization has been pushed to a reddish 

 yellow, or verges to a violet tint ; the shadows, however, are usu- 

 ally heavy, the fine lines are wanting, and the picture, if free from 

 stains, has a coarse appearance. 



Professor. Draper has noticed a fact respecting iodized plates, 

 which is essential to the success of the operation in this country. 

 It appears that the sensibility of the iodized surface to the action 

 of light, and the uniformity of that action, are increased bj^ keep- 

 ing the plate secluded from light for a certain period after iodiza- 

 tion. The duration of this period of seclusion depends on the 

 quality of the silver ; a French plate requires to be kept half an 



