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refraction is immediately calculated ; it is, in fact, equal to the thickness 
divided by the difference between the thickness and the increase of focal 
length. By this method Mr. Sorby has already determined the index of 
refraction of a large number of substances, and has obtained some interest- 
ing results. For example, he finds that the mineral called Proustite, or 
light-red silver-ore, has a refractive exponent as high as 3*28 ; much higher 
indeed than that of any substance previously examined. It has been pro- 
posed at various times to construct microscopic lenses of diamond or sap- 
phire, but although these gems have very high refractive indices, they are 
far exceeded in this respect by Proustite. In fact, were it possible to con- 
struct a lens of this rare mineral, it would have about twice the magnifying 
power of a lens of flint-glass of equal curvature. The index of flint-glass 
is certainly not higher than 1*6, which, as we have seen, is only about half 
that of Proustite. 
The Half-prism Spectroscope . — Instead of the ordinary isosceles prisms 
used in our spectroscopes, Mr. Christie, of Greenwich, has recently sug- 
gested the introduction of half-prisms. By a “half-prism” we understand 
an ordinary prism which has been cut in half by a plane perpendicular to the 
base. Such a prism possesses peculiar advantages. Suppose two slightly 
divergent rays of monochromatic light to pass through an ordinary prism, 
the angle of separation will not be altered, so that the rays emerge at the 
same angle to each other which they had at entrance. But if two such 
rays fall perpendicularly upon a face of the half-prism, the deviation of 
one ray in traversing the glass will be much greater than that of the other,, 
and they will consequently emerge separated by a much wider angle. In 
fact, the half-prism is really equivalent to a magnifier with cylindrical 
lenses. Mr. Christie has had several half-prism direct-vision spectroscopes 
successfully constructed by Mr. A. Hilger, and has described them before 
the Physical and Astronomical Societies (“Astron. Register,” May, 1877, 
p. 111). Half a prism of flint-glass is cemented to a half-prism of crown 
glass, and a combination giving great dispersion is thus obtained. If 
the double sodium line be viewed through such a spectroscope, the two 
lines will be seen widely separated, and in consequence of the dispersion 
they will be broader than with an ordinary instrument. With two such 
half-prisms a dispersion equal to that of ten compound prisms has been ob- 
tained ; and this, too, without any loss of definition. Moreover, either 
great dispersion or great purity of spectrum can be obtained at pleasure, 
according as the light is allowed to fall first upon the perpendicular or upon 
the oblique face of the half-prism. Hence by reversing a train of half- 
prisms, so that the light falls on the opposite side, a comparatively short 
spectrum is obtained, but one of great purity. Indeed, without use of col- 
limating lenses or slit, the sodium lines may be seen in an ordinary unsalted 
candle-flame, and Fraunhofer’s lines may be seen in the solar spectrum. 
The Otheoscope . — So many different forms have been given to the radio- 
meter by the ingenuity of Mr. Crookes, that at the Royal Society’s soiree in 
April he was able to exhibit as many as twenty modifications of the little 
instrument. Among these modifications were several so different from the 
earlier forms of radiometer, that Mr. Crookes 'proposes to distinguish them 
as OtheoscopeSj a name derived from a><9eco, I propel. In an ordinary radio- 
