POTASS I I'M. IHT.IIMrM. CAESIUM, AND LITHIUM 553 



which gives the rays a parallel direction. The rays of light having 

 passed through the slit, and having become parallel, are refracted and 

 dispersed in the prism, and the spectrum formed is observed through 

 the orifice of the other telescope B. The third tube D contains a hori- 

 zontal transparent scale (at the exterior end) which is divided into 

 equal divisions. The light from a source such as a gas burner or 

 candle placed before this tube, passes through the scale, and its image 

 is thus reflected on that face of the prism which stands before the tele- 

 scope B, so that the image of the scale is seen through this telescope 

 simultaneously with the spectrum given by the rays passing through 

 the slit of the tube C. In this manner the image of the scale and the 

 spectrum given by the source of light under investigation are seen 

 simultaneously. If the sun's rays be directed through the slit of the 

 tube C, then the observer looking through the orifice of B will see 

 the solar spectrum, and (if the aperture of the slit be narrow and the 

 apparatus truly adjusted) the dark Frauenhofer 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 Frauenhofer 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 Frauenhofer lines. Let us now 

 consider what will happen if the light from a flame coloured by various 

 salts be passed through the spectroscope. This is done by placing 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 invisible 

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



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 spectroscopic 

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

 looked for in special works on the subject. In tkis treatise 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 measures 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 lengths 

 of definite rays of light. 



K In order to give an idea of the several sizes of the scale',"we may observe that the 

 common spectrum extends from the zero 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 Frauenhofer line A (the extreme prominent line in the red) cor- 

 responds with the 17th division of the scale; the Frauenhofer 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 the 

 127th division of the scale. 



