CHAMBERS'S INFORMATION FOR THE PEOPLE. 



of light would always be accompanied by the pro- 

 duction of colour, and an achromatic lens, or lens 

 producing a colourless image, would be impossible. 

 But, by opposing a glass which bends five degrees 

 and disperses one degree to another glass which 

 bends three degrees and disperses one, the op- 

 posing dispersions will counterbalance each other, 

 and the refracted beam will be colourless. It is 

 on this principle that achromatic prisms and 

 lenses are made. The two prisms, A and B, 



Fig. 22. 



though drawn as separated from each other, are 

 firmly fastened by a colourless cement ; A is of 

 crown-glass, and B of flint-glass, and the angle 

 of B is so adjusted that the higher dispersive 

 power of flint-glass is counterbalanced by the 

 thinness of the prism, and a beam of white light 

 passing through the combination will be deflected, 

 but not decomposed into coloured beams. The 

 action of the achromatic lens, C, is precisely simi- 

 lar ; but from what has been said of the irrationality 

 of the spectrum, no combination of prisms or 

 lenses can be truly achromatic. The most that 



can be effected is to bring together any two rays 

 of the spectrum. 



When a pure spectrum of solar light is atten- 

 tively examined, and especially with a magnifying 

 eyeglass, it is seen to be crossed by many black 

 lines; and with the same prism, these lines occupy 

 a fixed position. Dr Wollaston, in 1802, was 

 the first to observe and describe these lines ; 

 but Fraunhofer, an optician of Munich, redis- 

 covered them in 1814, and published a beauti- 

 ful map with the positions of many of them 

 accurately marked. Fraunhofer discovered also 

 that dark lines existed in the spectra of many 

 different kinds of light, and that the lines of the 

 spectra of the planets and of the moon were the 

 same as those of the solar spectrum, while the 

 spectra of the fixed stars were all different. If 

 the source of light be a solid raised to a white 

 heat, a spectrum is obtained without any dark 

 lines, which is therefore continuous from end to 

 end ; if the source of light be a burning vapour, 

 there is a remarkable change in the spectrum. 

 Instead of being either continuous or crossed by 

 dark lines, it consists now of one or more bright 

 lines, which may be differently coloured. The 

 instrument by the aid of which these phenomena 

 may be most conveniently studied is called the 

 spectroscope, and consists essentially of, first, an 

 illuminated slit, from which parallel rays of light 

 proceed : secondly, a prism or train of prisms, to 

 separate the differently refrangible rays ; and 

 thirdly, a telescope, to view a magnified image of 

 the spectrum produced. The following figure 

 represents a section of the instrument in its 



Fig. 23. 



simplest form. AB is a brass tube, carrying at 

 one end, A, a metal piece with a slit, whose width 

 can be regulated by a fine screw, and at the other 

 end, a convex lens, B. The source of light, C, 

 is in the focus of the lens B, so that parallel rays 

 fall on the prism D, by which they are scattered 

 into the coloured beam, RGV. The telescope, 

 FG, is movable in the plane of the figure, so as to 

 describe part of a circle round the prism as centre, 

 and it can therefore be directed at will to any 

 part of the spectrum from R to V. The rays, such 

 as G, which fall on the lens G of the telescope, 

 are brought to a focus at H, where a distinct 

 image of this part of the spectrum is formed. 

 This image is in the focus of the eyeglass, F, so 

 that an eye placed at E sees it magnified. If the 

 part of the circle round which the telescope travels 

 is graduated, and furnished with an index or 

 vernier, the exact position of the telescope at any 

 time can be read off. But a simpler plan than 

 graduating the circle is to throw the image of an 

 illuminated transparent scale to the focus, H, of 

 the object-glass, 'G, so that the images of the scale 

 and spectrum are seen at the same time. This is 



248 



easily and ingeniously accomplished, thus : a glass 

 circular disc, S', has a small scale engraved in the 

 direction of a diameter, and all the rest of the disc 



Fig. 24, 



is blackened. This disc is fitted on to one end 

 of the tube ST at S ; and at the other end is a 

 lens, T, whose focal length is just the length of 



