180 Miscellanies. 



The concentration of the rays by a metallic mirror, so as to get quit 

 of the interference of the lens, would no doubt be a great improvement 

 in the camera obscura, provided it could be accomplished. May not 

 som.ething of this kind be the method followed by Daguerrein getting 

 his camera representations 1 



3. Preservation of the Impressions. 



It is evident that, as the impression is produced by the agency of 

 light on the compound of silver, when the paper is again exposed, the 

 light will begin to act, and ultimately darken the whole, thus effacing 

 the impression ; hence the necessity of a preservative process. Two 

 methods have been recommended by Mr. Talbot, as applicable to the 

 chloride, one by the iodide of potassium, the other by sea-salt. When 

 solution of iodide of potassium is added to that of lunar caustic, a yel- 

 low iodide of silver is thrown down. The same is the case when 

 the iodide is put on paper, previously covered with the chloride, and, 

 provided the solution is strong, it acts also on the chloride when dark- 

 ened, thus converting it to yellow iodide, which is not in the least af- 

 fected by light; hence, by putting the paper with the impression 

 through solution of the iodide, provided it is weak, the white chloride 

 only is acted on, and being converted to iodide, is no longer liable to 

 change. As, however, the iodide will act on the dark -chloride, it is of 

 the utmost consequence to attend to the strength of the solution, which 

 should be such that it will not attack the faint parts of the impres- 

 sion. After the paper is passed through it, it should be kept for some 

 time in water, to wash off the superfluous iodide of potassium, which, 

 if left on, would gradually destroy the whole of the impression ; in- 

 deed, even with this precaution, I find it extremely difficult to preserve 

 them. The second method recommended by Mr. Talbot is merely 

 immersing the paper in solution of sea-salt. This process does not, 

 however, seem to answer Avell ; I have repeatedly failed in preserving 

 the specimens in this way, and even when they are preserved, they 

 are completely altered in their appearance, and deprived of their 

 original brilliancy. 



I have already stated, that I prefer the phosphate of silver for ta- 

 king the impressions, not only because it is equally sensitive as the 

 chloride, but gives a greater variety of shades. In addition to these, 

 it has another advantage ; the impressions are easily preserved. Af- 

 ter various fruitless attempts, I at last found that the darkened phos- 

 phate is not soluble in ammonia, though, as is well known, the yellow 

 phosphate is easily dissolved. I had, therefore, recourse to this 

 for their preservation, and though I did not completely succeed at 

 first, yet I at last did so, by attending to the precaution of washing off 

 he ammoniacal solution, because, when left on, the impression gradu- 



