LIGHT AND SOUND. 



the tube. The scale, S, is illuminated by a light, 

 L, placed behind it ; and by slightly drawing out 

 or pushing in the cap to which the scale is 

 attached, the rays are made to emerge from the 

 lens T, so that, after being reflected by the face of 

 the prism, they are focused at H. Of course the 

 direction of the scale-tube and 

 that of the telescope must be 

 equally inclined to the reflecting 

 face of the prism, and therefore 

 the former is movable round the 

 same centre as the telescope, 

 and in the same plane. The next 

 figure gives a view of a com- 

 plete two-prism spectroscope, 

 and the use of its various parts 

 will be readily understood. 



When the solar spectrum is 

 examined with such an instru- 

 ment, the positions of the dark 

 lines are as shewn in fig. 26 ; 

 but a more powerful instrument 

 reveals the existence of several 

 thousands. Fraunhofer mapped 

 576, and designated the principal 

 ones by the letters A, B, C, D, E, 

 F, G, H, by which designations 

 they are still known. Their po- 

 sitions are determined either by 

 stating the index of refraction 

 for each line, or, which amounts 

 to the same thing, the length of the ethereal wave 

 which produces each ray. The latter method is 

 preferable, as being independent of the nature of 

 the prisms employed ; thus the wave-lengths which 

 would produce rays at A and the first thick line 

 of the group H are respectively 7604 and 3968, the 

 unit being the thousand-millionth part of a metre. 

 Suppose, now, our source of light to be an incan- 

 descent gas or vapour, which may be effected by 



i exposing a substance to a heat which first vola- 

 ! tilises it and then burns the vapour. The pale flame 

 of a Bunsen gas-lamp will serve to volatilise and 

 burn many substances. We find that not only do 

 the spectra consist of isolated bright lines, but that 

 each substance has aline or set of lines peculiar to 



Fig. 25. 



itself. Burning sodium vapour gives two bright 

 lines identical in wave-length, and therefore, when 

 using the same prism, in position with the two 

 dark lines, D, of the solar spectrum. When the 

 electric spark is passed through a closed tube con- 

 taining hydrogen, three bright lines, exactly corre- 

 sponding to C, F, G, are obtained. So certain are 

 the results, that after we have mapped the bright 

 lines of each substance, we can tell, on looking at 



OR AN. VEL. 



e 6 



Fig 26. 



any such spectrum, what the burning substance 

 must be. If two or more different bodies be burn- 

 ing together, the different sets of lines are seen at 

 the same time, and no line of any one set interferes 

 or coincides with a line of another. It will now be 

 seen how readily this method lends itself to che- 

 mical analysis, for if in the spectrum of any com- 

 pound substance we observe the lines of a particular 

 element, we are certain that that element exists in 

 the compound. And so delicate is this mode of 

 analysis, that if a single grain of soda be dissolved 

 in 2500 gallons of pure water, it can be infallibly 

 detected. Again, if we find in a spectrum bright 

 lines that have not previously been observed, they 

 indicate the presence of a new element. In this 

 way, Bunsen discovered the two new elements, 

 caesium and rubidium; Crookes, thallium; and 

 Reich and Richter, indium. The connection between 

 the dark lines of the solar spectrum and the bright 

 lines of the elements was first shewn clearly by 



Kirchhoff in the year 1859. By allowing sunlight 

 | to enter one half of the length of the slit of his 

 j spectroscope, and the light from a sodium flame 

 the other half, he not only shewed that the dark 

 lines, D, of the former coincided perfectly with the 

 bright lines of the latter a fact which had fre- 

 quently been asserted before but he went much 

 further. Having the continuous spectrum of the 

 lime-light in one part of the field of view of his 

 telescope, and the two bright lines from burning 

 sodium immediately above it> he found that when 

 the lime-light shone through the burning sodium 

 vapour, the two bright lines were instantly con- 

 verted into two dark lines in the very same place. 

 He explained this by supposing that burning 

 sodium vapour which could emit light waves of a 

 certain length, could also absorb precisely those 

 waves, when waves of all lengths were sent 

 through it. He found the same thing to hold for 

 many other substances which he tried, and was 



249 



