CHEMICAL ACTION OF THE SOLAR RADIATIONS. 147 
equally remarked the differences produced on actinic power by colourless 
screens. 
The exalting or depressing power of certain media was also particularly 
examined by Sir John Herschel, who observed very early in the progress of 
his inquiries, that if a piece of thin post-paper, merely washed with nitrate 
of silver, was exposed to clear sunshine, partly covered by and strongly 
pressed into contact with glass, and partly projecting beyond it, the portion 
under the glass was very much more affected than the part exposed, it being 
often blackened, in the same time, to a tone which would require at least three 
times the exposure uncovered. In practice photographers have availed them- 
selyes of this, and it is usual to place the prepared paper in the camera behind 
a plate of glass. 
The philosophy of this is ill understood: it has been thought to be due to 
the circumstance, that the most transparent glass abstracts a portion of Licur, 
and thus leaves the actinic power more free to.act on the sensitive material. 
The entire question demands a more attentive examination than it has hitherto 
received. ‘The peculiar action of coloured media is more accurately defined ; 
and as a knowledge of the influences exerted will have its value in guiding 
new observers, the more decided and peculiar cases of obstruction to the ac- 
tinic radiations must be given. It cannot however be too strongly impressed, 
that every variety of glass or fluid media employed should be submitted to 
prismatic examination, since the colour alone is not a guide to the quality of 
the radiations permeating a particular medium. 
Supposing the effect of exposure of a standard photographic preparation to 
the direct solar radiations in a given time to be represented by 100, the action 
produced by the interposition of coloured media is relatively shown in the 
other numbers.’ Although many specimens of blue glass show an exalting 
effect, and consequently should be represented by a number in excess, it is 
thought advisable to regard them as equal to unshaded exposure. 
Glasses. 
@eapoeure to vinshaded sunshine.............. 220025 κοι σε νιν es +100 
Ruby glass coloured with oxide of gold which insulates perfectly 
SE EY Si ας ΒΕ SP oR rr Pes - τὰ -- 25 
Brown red coloured with copper, which admits the permeation of 
all the rays below the orange, and a faint line of blue .... — + 30 
Orange glass coloured with iron, cutting off the violet, indigo, and 
MRE NC LAV Lie oa τις διε «eet edie καὶ 38 ale — 10 
Lemon-yellow glass, probably coloured with iron, reducing the 
spectrum to three patches of blue, red and yellow ........ — 8 
Yellow glass stained with oxide of silver.................-4- — 3 
Green glass,—a deep pure green produced by oxide of copper .. + 74 
Blue glass, cobalt, obliterating all the mean luminous rays, and 
exhibiting the extreme red in great purity ..........04:- +100 
Fluids. 
Red.—Carmine dissolved in ammonia ; --cutting off all the rays above 
the red, except when in very thin layers it admits a small line of 
ICR CARO OS yo ee ea are — 20 
Orange yellow.—Solution of bichromate of potash with a little sul- 
phuric acid ; giving but a trace of the blue rays, all the least re- 
frangible being well-defined ».5.,...ine-sseseeenssteecues - 7 
Lemon yellow.— Quadro-sulphuret of lime of Dalton ;—cutting off all 
the prismatic rays above the inner limits of the blue ........ οὐ 
