662 PRINCIPLES OF CHEMISTRY 



sun's rays be directed through the slit of the tube C, then the ob- 

 server looking through the eye-pie.ce of B will see the solar spectrum, 

 and (if the aperture of the slit be narrow and the apparatus cor- 

 rectly adjusted) the dark Fraunhofer lines in it. 24 Small-sized 

 spectroscopes are usually so adjusted that (looking through B) the 

 violet portion of the spectrum is seen to the right and the red portion 

 to the left, and the Fraunhofer line D (in the bright yellow portion of 

 the spectrum) is situated on the 50th division of the scale. 25 If the 

 light emitted by an incandescent solid for example, the Drummond 

 light be passed through the spectroscope, then all the colours of the 

 solar spectrum are seen, but not the Fraunhofer lines. To observe 

 the result given by a flame coloured by various salts a Bunsen gas 

 burner (or the pale flame of hydrogen gas issuing from a platinum 

 orifice) giving so pale a flame that its spectrum will be practically invisible 

 is placed before the slit. If any compound of sodium be placed in the 

 flame of the gas burner (for which purpose a platinum wire on whose end 

 sodium chloride is fused is fixed to the stand), then the flame is coloured 

 yellow, and on looking through the spectroscope the observer will see a 

 bright yellow line falling upon the 50th division of the scale, which is 

 seen together with the spectrum in the telescope. No yellow lines 

 of : other refractive index, nor any rays of any other colour, will be 

 seen, and, therefore, the spectrum corresponding with sodium com- 

 pounds consists of yellow rays of that index of refraction which belong 

 to the Fraunhofer (black) line D of the solar spectrum. If a potassium 

 salt be introduced into the flame instead of a sodium salt, then two 

 bands will be seen which are much feebler than the bright sodium 

 band namely, one red line near the Fraunhofer line A and another 

 violet line. Besides which, a pale, almost continuous, spectrum will be 



24 The arrangement of all the parts of the apparatus so as to give the clearest possible 

 vision and accuracy of observation must evidently precede every" kind of spectroscopio 

 determination. Details concerning the practical use of the spectroscope must be 

 looked for in special works on the subject. In this trea ise the reader is supposed to 

 have a certain knowledge of the physical data respecting the refraction of light, and its 

 dispersion and diffraction, and the theory of light, which allows of the determination 

 of the length of the waves of light in absolute measure on the basis of observations 

 with diffraction gratings, the distance between whose divisions may be easily measured 

 in fractions of a millimetre ; by such means it is possible to determine the wave-length 

 of any given ray of light. 



25 In order to give an idea of the size of the scale, we may observe that the 

 ordinary spectrum extends from the zeto of the scale (where the red portion is situated) 

 to the 170th division (where the end of the visible violet portion of the spectrum is 

 situated), and that the Fraunhofer line A (the extreme prominent line in the red) cor- 

 responds with the 17th division of the scale ; the Fraunhofer line F (at the beginning of 

 the blue, near the green colour) is situated on the 90th division, and the line G, which is 

 clearly seen in the beginning of the violet portion of the spectrum, corresponds with tha 

 127th division of the scale. 



