1864. | Physics. 501 
belonging more to the domain of chemistry than optics; one com- 
pound, the silico-tungstate of soda, is likely to be of great use in the 
manufacture of fluid prisms, inasmuch as a solution can be obtained 
having the specific gravity of 3-05, and being very fluid, but on which 
glass, quartz, and most stones will float. If its refracting powers are 
equal to its density, this solution will be invaluable for fluid prisms. 
We may also mention that a compound of thallium with the elements 
of alcohol, ethylate of thallium, has also been proposed for the con- 
struction of fluid prisms. It is a heavy, oily liquid, of about the 
density and refracting power of bisulphide of carbon, but unlike the 
latter liquid, non-volatile. 
A most ingenious application of scientific principles to the illumi- 
nation of theatres has just been carried out by M. Soubra. The foot- 
lights in front of the stage of a theatre are almost invariably argand 
burners, surrounded by glass; not only is there very great danger of 
the thin dresses worn by the actresses taking fire, but the products of 
combustion vitiate the atmosphere of the stage, whilst the heated air 
rising from them just across the line of sight of the spectators in the 
stalls, renders the view from these seats less pleasant than it would 
otherwise be. The reason why the flame of an argand burner, or any 
any other light, points upwards, is owing to the heated air and pro- 
ducts of combustion being lighter than cold air; the former, therefore, 
rise upwards, and cause the flame to rise also. If, however, a down- 
ward movement could be impressed upon the heated products of 
combustion, the flame would equally well follow the same direction, 
and would continue to burn downwards. M. Soubra, therefore, takes 
a wide glass pipe, bent in the form of the letter U ; one leg, however, 
being considerably longer than the other one. Just inside the 
shorter leg of the two, an argand burner is inverted, and the longer 
leg of the tube being heated for a short time, so as to rarefy the air 
in it, and cause a downward current in the short end, the argand 
burner is lighted, and the flame, following the direction of the 
current of air with which it is surrounded, continues to burn upside 
down—the current once established being sustained by the heat from 
the inverted flame. The advantages from this new arrangement are 
as follows:—The supports of the globes, or lamp-glasses, are placed 
above the flame, and do not intercept the light ; the reflectors also are 
in no danger of becoming blackened by smoke, and they collect rays 
that would otherwise be lost in the air; the flame has a more elevated 
temperature, on account of the heat being concentrated by the syphon, 
and the carbon is consequently rendered more incandescent ; the pro- 
ducts of combustion may easily be carried away through the longer leg 
of the tube into a chimney, instead of vitiating the air of the apart- 
ments. The advantages as to safety, &e., of this plan are so obvious, 
that no time should be lost in introducing this method of illumination 
in this country. It is, we understand, already adopted in France with 
great success. 
Many years ago Mr. Fox Talbot discovered that when a continuous 
spectrum is examined by covering one-half of the pupil of the eye 
