Mechanics of Luminosity . 267 



spectra of different orders. This is also confirmed by a 

 simple experiment. If an equality of brightness between the 

 arc-light and the amyl-acetate light had been established in 

 the first spectrum by the use of clouded glasses, this equality 

 was also observed in the second and third spectra. 



The comparison in brightness between the arc lamp and 

 amyl-acetate lamp was made at a point a little distance from 

 the yellow, on the side of the green, so that it might not be 

 disturbed by sodium contained in the arc. The results of 

 experiment are as follows : — The breadth of a sodium line is 

 i of the distance between the two lines. The brightness 

 diminishes from the centre rapidly, then more slowly, and at 

 the edges more rapidly again. The measurements were made 

 at about J of the breadth from the edge after the brightness 

 of the arc-light had been made equal to that of the sodium flame 

 by rotation of the Nicol, it had then to be reduced to ^ to be of 

 the same brightness as the amyl-acetate lamp. 



The brightness of our sodium lamp therefore for rays 

 situated about £ of the breadth of a sodium line from its 

 edge is 34 times greater than that of the amyl-acetate lamp 

 at the corresponding point. 



We may now apply the above results to the experiments 

 on p. 261, where the brightness of the amyl-acetate lamp was 

 compared with that of the sodium flame. 



We will take i' to refer to the sodium flame, and i' f to the 

 amyl-acetate lamp. In the sodium double lines the breadth 

 of a single line is J of the distance between the centres of 

 the lines, so that the total breadth of the two together is \ 

 this distance ; the distance of the centres is known to be 

 0*0006 //,, consequently the quantity \ 2 — ^i =0*0003 fi; further 

 V was found to be 2*7, since A=0*0176/-t (p. 259) therefore 

 ' v _ 2 . 77; , 0-0176 _ 

 i"~ l " x 0-0003 - lb ^ 

 The direct determination for a point about J from the edge of 



i' 

 the weaker line gave -j t = 34. It follows from the difference of 



these numbers, which agree as to order of magnitude, that the 

 above assumption as to the distribution of brightness is not 

 strictly correct, but that the brightness of a sodium line must 

 increase rapidly in the centre, which is also confirmed by the 

 appearance to the eye. This further indicates the possibility 

 that in spite of the breadth of the spectral lines, interferences 

 may occur with differences of path up to more than 100,000 

 wave-lengths, such as have been observed by several expe- 

 rimenters. [To be continued.] 



