244 Spectroscope Apparatus. 



the light ray, or arises from any other source, has disturbed 

 its homogeneous character. This instrument is described by 

 Mr. Gassiot in Proceedings of the Royal Society, No. 63. The 

 prisms are formed with a refracting angle of 50°, and con- 

 sequently eight prisms, with the usual arrangement, 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 6°, the angles being 

 arranged in the contrary 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 pass- 

 ing 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 colour. But if a ray is imperfectly 

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

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

 is complete. 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 com- 

 plete, the intermediate tints are innumerable, and one passes 

 into the other by insensible gradations. The artistic effect of 

 a slightly dispersed spectrum is a brilliant but violent contrast 

 of dissimilar colours. That of the highly dispersed spectrum 

 is a harmonious juxtaposition of all the colours 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 per- 

 ceive 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 refrangible. 

 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, 

 obtained from the horse chesnut, or with an alcoholic solution 

 of stramonium, they are sufficiently altered in refrangibility to 

 become visible, 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. Brown- 

 ing worked it into a prism of 60°, with 2i-inch sides. 



