RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 39 
addition of a pale yellow glass to such a medium eliminates these, and converts the 
combination into a pure red. But the photographic character of brown and yellow 
media is far from conforming itself universally to this indication. Of this the com- 
mon yellow and brown glasses which are met with in the shops afford a striking ex- 
ample. The spectra impressed by transmission through such glasses consist of two 
distinct portions, separated by a larger or shorter interval, and having themselves a 
less or greater extent and intensity in proportion to the depth of tint of the glass. 
If that tint be only a pale yellow or straw colour, both portions are finely developed. 
If in that case the exposure be long continued, the continuity of the spectrum is 
maintained by a feeble train of darkness in their interval. Abstraction made of this, 
the most refrangible portion may be stated to commence precisely at the end of the 
violet, or at + 40'6, whence it increases gradually to a maximum at + 60, and thence 
dies gradually away, till it ceases to be traceable at + 85, giving a range of forty-four 
parts in the ultra-violet rays for its extent. The interval between the portions mea- 
sured in the same experiment from which the foregoing numbers are taken, and a 
similar abstraction made of the faint connecting train of darkness, was twenty-three 
parts, occupying the whole space, which in a complete spectrum would have been 
filled with violet and indigo light. The least refracted portion, commencing at -f- 18, 
rapidly attained the full intensity it would have had if freely exposed without the 
yellow glass, and from thence to — 11, where it terminated, offered the same suc- 
cessive tints (dark blue, green, and red, the paper having been prepared with muriate 
of soda) which it would have done in those circumstances. As to the tint of the ultra- 
violet portion, it was a fine violet-purple without any variation. 
104. When the colour of the glass used was a pretty strong brown, such as to cut 
off all the violet, indigo, blue, and the greater part of the green, the ultra-violet por- 
tion of the photographic spectrum, though much enfeebled and contracted, was still 
distinctly traceable from + 58 to -f- 83, the maximum (at -f- 70) being considerably 
more remote than in the former case, and this might be owing to an actual difference 
in the colouring matter of the glasses. On the other hand, the less refrangible por- 
tion had shrunk to a small and well-defined oval spot, extending from + 3*5 to 
— 9'5, or over thirteen parts, or rather less than two diameters of the sun, having its 
upper part coloured lead-grey, the lower red. 
105. One of the finest and most brilliant yellow liquids with which I am acquainted 
for purposes of absorptive analysis, is the solution of that substance to which Dalton 
has given the name of quadro-sulphuret of lime, and which is easily prepared by boiling 
lime and sulphur together in plenty of water. Its transparency is so great and its 
tint so luminous a yellow, that it scarcely seems to impair the brightness of white 
objects seen through it in thicknesses of an inch or two. But its action on the more 
refrangible end of the spectrum is very energetic, and at a thickness = T1 inch was 
found to be such as to limit the visible spectrum to an extent of 33’2, including the 
two terminal semidiameters of the sun. Only a faint trace of blue was visible at the 
