Mr Davidson on the Process of Daguerreotype, <$r. 179 



and here I entirety differ from Daguerre. He recommends 

 the mercury to be raised to about 160°, and then to take away 

 the lamp till the mercury fall to about 1.30°. This will doubt- 

 less give a picture, but not a clear and brilliant one. I raise 

 the mercury to about 165°, or from that to 170°. About 10 

 minutes should be occupied in raising the temperature, and 

 about 10 or 15 in letting it fall, in order to produce a brilliant 

 picture. The operation may be sooner completed by raising 

 the mercury higher, but the picture would be harsh and par- 

 tially blighted. The only way by which I have been able to 

 obtain a good picture has been by finding what length of ex- 

 posure in the camera was required to cause the picture appear 

 faintly when the mercury was raised to 165° or 170°, and 

 then, while the mercury was falling, by allowing it the time 

 already stated, the picture came oat with great force and bril- 

 liancy, free from the disagreeable dark blue tint which is in- 

 variably found in the French Daguerreotypes, and always will 

 be by the process in the mercury, as described and recom- 

 mended by Daguerre. But it must be observed that, if the 

 picture has been kept too long in the camera, it will never 

 stand the proper action of the mercury, but will be completely 

 spoiled. 



The last part of the process is to remove the iodine from 

 the picture by washing it with a solution of common salt, or 

 the hyposulphite of soda, and then with pure water. 



If the plate has been exposed to the action of mercury pre- 

 viously to its being polished, it must be heated as recommend- 

 ed by Daguerre, but I polish it, as before, without oil. 



Having thus given a brief description of the process of Da- 

 guerreotyping, I shall next shew the action of light in the 

 camera, and the method by which a fiat and large picture is 

 obtained, pointing' out the errors that still exist in the Daguer- 

 reotype camera, and endeavouring to demonstrate that the 

 error in the lenses will be in Paris only the one-half of what 

 it is here, and that the Parisians ought, therefore, to produce 

 superior impressions. It must be obvious to all who are ac- 

 quainted with the science of optics, that the lens in the Da- 

 guerreotype camera is not only placed in its worst position 

 with respect to spherical aberration, but that the focus of the 



