OPTICS. 119 
base. Upon the two first surfaces lay plates of ground plate glass, which 
may be kept firm by a brass clamp; fill the hollow prism thus formed with 
the liquid in question, through the small hole, and in it insert a stopper of 
ground glass. Fig. 30 represents a prism of this character, consisting of 
two hollow prisms close to each other. Another form of the hollow prism 
is shown in fig. 81. A three-sided prism of brass, or still better, of glass, is 
bored through, either, as in the figure, by a quadrangular, or by a round 
aperture; upon the two refracting surfaces plates of glass are laid, which 
may be pressed upon the surface of the hollow prism by means of. four 
screws. Above is the aperture through which the prism may be filled, and 
which is then to be closed. 
If a ray of light pass through a plate, as of glass, with parallel sides, or 
through several superimposed plates of different materials (fig. 82), it 
emerges in a direction parallel to the original one, though somewhat 
displaced from it. 
The refractive power of a body is equal to n*—1, or the square of the 
exponent of refraction, with respect to a vacuum minus unity ; the quotient 
of the refracting power, divided by the density, rt ane sf is called the 
absolute refracting power. 
Arago, Biot, and especially Dulong, have instituted very accurate 
experiments with regard to the refractive indices of gaseous bodies; they 
have discovered that the refractive powers of gases are proportional to their 
densities. Dulong’s experiments had particularly for their object the 
comparison of the refractive powers of gases at equal pressures and 
temperatures. ‘To give them such a density as to produce precisely the 
same deviation, he employed a prism whose refracting power amounted to 
145°, standing in connexion with a reservoir, 7 ( pl. 20, fig. 31), and which 
could be exhausted on one side by connexion with an air-pump, and filled 
with gas on the other. He filled the prism first with dry air of the pressure 
and temperature of the atmosphere, and sighted then with a telescope set up at 
some distance, towards the image of a distant point refracted by the prism. 
The prism was then exhausted without disturbing it, and filled with another 
gas. By changing the pressure he could bring the refracted image of the 
same point of sight into the same part of the field of the telescope as before. 
Now, supposing carbonic acid gas to be compared with dry air, and that the 
pressure amounted to 18.9 inches, it is evident that as the pressure under 
which an equal deviation took place in air amounted to twenty-nine inches, 
under the circumstances the indices of refraction and the refracting power 
itself must be the same in air, that is, 18.9:29::1:2,; hence we obtain 
x = 1.53 as the index of refraction of carbonic acid at an atmospheric 
pressure of twenty-nine inches. 
The refraction of light through lenses is of especial practical interest. Of 
these lenses the most important are the spherical, bounded either by portions 
of spheres, or by these and plane surfaces combined. Six kinds of spherical 
Jenses are distinguished in optics, all of them represented in fig. 32: 
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