1866. } Physics. 297 
tive degrees of sensitiveness observed when the several papers were 
referred to a normal tint; thus :— 
Chloride of silver paper .« : : : 10090 
Chloro-iodide of silver : ‘ 5 - 1078 
Chloro-bromide of silver . , . P 4022 
Bromide of silver ; , 3 é 2396 
Bromo-iodide of silver : : . 4 4060 
M. Poitevin has lately succeeded in producing photographs on 
Ls in their natural colours. He prepares his sensitive paper in 
the following way :—Havying obtained a layer of violet subchloride 
of silver on the paper, by the action of light on the white chloride 
in the presence of a reducing agent, he applies to the surface of the 
paper a liquid composed of one volume of a saturated solution of 
bichromate of potash, one volume of saturated solution of sulphate 
of copper, and one yolume of a solution containing five per cent. of 
chloride of potassium. ‘This paper is dried and kept in the dark ; 
it will keep good for several days. In this mixture the bichro- 
mate of potash is the principal agent, the sulphate of copper facili- 
tates the action, and the chloride of potassium preserves the whites 
which are formed. In copying paintings on glass, the exposure tq 
direct light need only last five or six minutes; but the time must, 
to some extent, depend on the transparency of the picture to be 
copied, and it is easy to watch the development of the image on the 
paper. The paper is not sufficiently sensitive for use in the camera, 
To preserve the pictures it is only necessary, first, to wash them 
with water acidulated with chromic acid, then to treat them with 
water containing bichloride of mercury, afterwards with a solution 
of nitrate of lead, and, lastly, well wash them with water. After 
that they will not change in ordinary light, but will, however, turn 
brown in direct sunlight. 
In order to prepare a window for the illumination of a photo- 
grapher’s dark-room, Obernetter mixes an acid solution of sulphate 
of quinine with some gum or dextrine, and paints the mixture over 
a thin sheet of white paper. With this he covers the window-panes, 
and he states, that on the brightest day a window so prepared will 
allow no actinic light to pass. This we doubt. The strongest 
solution of sulphate of quinine allows some of the photographic 
rays of light to pass through, and it is not likely that paper coated 
with the salt would be more impervious to the actinic rays than 
the solution itself. 
The micro-spectroscope has received its first application to 
medico-legal purposes in the examination for blood-stains of the 
hatchet supposed to have been used in the Aberdare murder. Dr. 
Bird Herapath, F.R.S., who was retained by the Crown, placed 
sections of the handle in distilled water, and submitted pe solution 
x 
