1864. | Optics. 345 
and nitroprusside of sodium in ten times their weight of water, and in 
this manner forms a liquid which is highly sensitive to light, deposing 
Prussian blue as a precipitate under the influence of luminous action, 
whilst it remains quite clear in darkness. He therefore prepares this 
liquid in obscurity, and takes its specific gravity. After exposure to 
the light he filters off the precipitated Prussian blue, and again takes 
the specific gravity of the clear liquid. It will of course have dimin- 
ished in density by the amount of solid matter separated, and the 
difference between its former and latter specific gravity will represent 
the chemical action, the numbers obtained varying directly with the 
intensity of the light. 
The Electric Light appears to have permanently taken its place 
amongst theatrical properties. In Paris, where more attention is 
paid to scenic effects than in this country, the celebrated optician 
Duboscq has devised some marvellous imitations both of lightning and 
of the rainbow. 'The former is obtained by a concave mirror, in the 
focus of which are the two carbon poles of a powerful battery nearly 
in contact, and so adjusted that when the mirror is rapidly moved in the 
hand the poles are caused to touch for a brief interval, and flash a daz- 
zling beam of light across the stage. The zigzag efiect of lightning, and 
its peculiar blue colour, are very perfectly imitated by this means. But 
more wonderful than this is therainbow. Inthe representation of the 
opera of Moise it is requisite in the first act to introduce a rainbow, 
and this has hitherto been effected either by painting or by projecting 
the image on the scene from a magic lantern by means of a coloured 
slide. In the latter case the stage had to be darkened in order to allow 
the rainbow to be seen, and this of course destroyed the illusion. M. 
Duboscq, by a happy modification of his spectrum apparatus, and by 
employing a curved instead of astraight sht, and a small-angled prism, 
has succeeded in projecting the very brilliant electric spectrum on the 
scene, with the proper curvature and the identical colours of the real 
rainbow, and this of such a vividness that it is plainly visible in the 
full light of the stage. In these days of sensation-spectacles we feel 
confident that a real rainbow on the stage would attract quite as crowded 
houses as a “ tremendous header,” and it is somewhat surprising that 
no manager thought of introducing so novel an effect last Christmas. 
If our Continental neighbours have not yet supplied us with all 
their electric effects, they have not hesitated to make full use of the 
Dirksian ghost, which has so long reigned unrivalled at the Polytechnic 
under the energetic management of the director, Mr. Pepper. In the 
last act of the ‘ Seeret de Miss Aurore, as performed at the Theatre 
Impérial du Chatelet, the ghost of Conyers is made to confront his 
assassin, Softy, with incorporeal bank-notes in his hand, and poetic jus- 
tice is supposed to be avenged by the horror which seizes the murderer 
when he finds himself unable to grasp them. But the head-quarters of the 
ghostly illusion are at the Séances of M.Robin, perhaps the most scientific 
of modern followers of Cagliostro. In availing himself of the now well- 
known machinery necessary to produce the ghost illusion, he combines 
the experience of a wizard with the appliances of a man of science, and 
