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 
something of this kind be the method followed by Daguerre in getting 
his camera representations? 
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, itis 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 impre> 
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 5 © 
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 nob 
however, seem to answer well; 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 1a 
king the impressions, not only because it is equally sensitive a the 
chloride, but gives a greater variety of shades. In addition to 
it has another advantage ; the impressions are easily preserved. 
ter various fruitless attempts, I at last found that the darkened pa 
0 
this 
phosphate is easily dissolved. I had, therefore, recourse we! “ 
for their preservation, and though I did not completely meee 
first, yet Lat last did so, by attending to the precaution of washing 
he ammoniacal solution, because, when left on, the impression ; 
these, 
