266 ANNUAL OF SCIENTIFIC DISCOVERY. 



becomes very bad as soon as the heat which accompanies the light ray, 

 or arises from any other source, has disturbed its homogeneous charac- 

 ter. This instrument is described by Mr. Gassiot in Proceedings of 

 the Royal Society, Xo. 63. The prisms are formed with a refracting 

 angle of 50, and consequently eight prisms, with the usual arrange- 

 ment, would " cause a ray of light to travel more than a circle." In 

 order to employ eleven, Mr. Browning, instead of making the outer 

 sides of each hollow prism flat, formed them of crown glass prisms, 

 having a refractive angle of!) , the angles being arranged in the con- 

 trary direction to those of the fluid prisms. These sides take off very 

 little of the dispersive power of the bisulphide, and enable eleven 

 prisms to be employed. 



The dispersive power of glass or any other substance depends upon 

 the different refrangibility of the different rays that make up white 

 light. A beam of white light contains an infinite number of rays of 

 all hues, from red to violet (besides rays invisible to man). Each 

 ray takes its own bend on passing through the prism, and is thus 

 more or less separated from other rays that are bent in a different 

 degree. When this dispersive process has completely separated any 

 ray from its companions, passing it through another prism simply 

 refracts it, without any action on its color. But if a ray is imper- 

 fectly separated from adjacent rays in the scale, the further action of 

 one or more prisms carries on the work of separation until it is com- 

 plete. In a spectrum formed by slight dispersion the red, yellow, 

 green, and blue are seen in bands that contrast strongly with each 

 other. When the dispersion is more complete, the intermediate tints 

 are innumerable, and one passes into the other by insensible grada- 

 tions. The artistic effect of a slightly dispersed spectrum is a bril- 

 liant but violent contrast of dissimilar colors. That of the highly 

 dispersed spectrum is a harmonious juxtaposition of all the colors in 

 the scale. 



As the human ear cannot hear all sounds, so the human eye cannot 

 see all rays of light. There may be sounds too sharp, consisting of 

 vibrations too rapid, for the ear to perceive them, and sunlight has 

 rays which must undergo a change before our eyes can bring them to 

 a focus. Glass absorbs many of these rays, which are highly refrang- 

 ible. Quartz transmits them, and hence a prism of that substance 

 adds them to the spectrum which it forms. If such a spectrum is 

 received on a screen prepared with a substance called cesculin, ob- 

 tained from the horse-chestnut, or with an alcoholic solution of stra- 

 monium, they are sufficiently altered in refrangibility to become vis- 

 ible, and a beautiful blue addition to the violet end of the spectrum is 

 seen. It is very difficult to find a large quartz crystal that can be cut 

 into a good prism, but Mr. Gassiot obtained a splendid one from 

 Japan, and Mr. Browning worked it into a prism of 60, with 2^-inch 

 sides. 



The different spectroscopes to which we have alluded are all con- 

 structed upon the principle of indirect vision. That is to say, the 

 observer does not point the telescope straight at the light he wishes to 

 see. It reaches him round the corner through the refraction of the 

 prism. This plan is handy enough for fixed apparatus, but for rapid 

 examination of the light that comes from different portions of the sky, 



