and its application to talcing Portraits from the Life. 219 



it is to be heated and washed with nitric acid, as indicated in 

 the French account, and finished by being rubbed with whiting 

 (creta prceparata), in the state of a very dry powder, going 

 over it for the last time with a piece of clean dry cotton ; this 

 gives an intensely black lustre, which cannot be obtained by 

 rottenstone alone, and thoroughly removes any film which 

 nitric acid may have left. 



To coat with iodine, I make use of a box about two inches 

 deep, in the bottom of which that substance in coarse flakes 

 is deposited ; no cloth intervenes, but the silvered plate, with a 

 temporary handle attached to it, is brought within half an inch 

 of the crystals, and it becomes perfectly coated in the course 

 of from one to three minutes; no metallic strips are necessary 

 to ensure 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. (i Be- 

 yond 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 obser- 

 vation must be received with some limitation. A plate, which 

 has been iodized, does not appear so quickly to lose its sen- 

 sitiveness. On the other hand, by keeping it in the dark for 

 twelve or twenty-four hours, its sensitiveness is often remark- 

 ably increased. Other advantages also accrue. Those who 

 have made many of these photogenic experiments, will have 

 had frequent occasion 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 correspondingly, 

 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 importance to the travelling daguerre- 

 otyper ; he will find that the iodine does not lose its sensi- 

 tiveness in many days. 



In a paper read before the Royal Society, of which an abs- 

 tract is given in the April number of this Journal for the 

 present year (p. 333.), Sir John Herschel states, that there 

 is an absolute necessity of a perfect achromaticity in the ob- 

 ject-glass of a photographic camera. M. Daguerre appears 

 to have been under the same impression, and recommends in 

 his published account such an object-glass. 



