MEMOIR XIV.] THE PROCESS OF DAGUERREOTYPE. 209 



silvered plate, with a temporary handle attached to it, is 

 brought within half an inch of the crystals, and it be- 

 comes perfectly coated in the course of from one to three 

 minutes ; no metallic strips are necessary to insure this 

 effect ; if the edges and corners are thoroughly clean, the 

 golden hue will appear uniformly. 



M. Daguerre recommends that the plate, after being 

 iodized, shall be placed in the camera without loss of 

 time. The longest interval, he says, ought not to exceed 

 an hour. " Beyond this space the action of the iodine 

 and silver no longer possesses the requisite photogenic 

 properties." 



There may be something peculiar in the preparation 

 of the plate as I have described it, but it is certain that 

 this observation must be received with some limitation. 

 A plate which has been iodized does not appear so 

 quickly to lose its sensitiveness. On the other hand, by 

 keeping it in the dark for twelve or twenty-four hours, 

 its sensitiveness is often remarkably increased. Other 

 advantages also accrue. Those who have made many of 

 these photogenic experiments will have had frequent oc- 

 casion to remark that the film of iodine is not equally 

 sensitive all over, that there are spots or cloudy places 

 which do not evolve any impression, and often the whole 

 is in that condition that the bright parts alone come out, 

 while the parts that are in shadow do not evolve corre- 

 spondingly, nor can they be well developed, except at the 

 risk of solarizing the picture. Now a plate that has 

 been kept for several hours is by no means so liable to 

 these effects: I do not pretend to give any reason for 

 this, but merely mention it as a fact, of considerable im- 

 portance to the travelling daguerreotyper ; he will find 

 that the iodine does not lose its sensitiveness in many 

 days. 



In a paper read before the Royal Society, Herschel 



O 



