162 Prof. E. W. Wood. 



With this flame as a dispersing piece the photographs shown in 

 Plate 2, fig. 8, were obtained. In comparison with some of the other 

 photographs this example is very poor, and I have included it merely 

 to make the set complete. An exposure of about 10 minutes was 

 necessary, with an " Erythro " plate. Fig. 9 was taken with one of 

 the dispersion tubes, and shows what may be called the second stage. 

 After the exposure was over, but before the plate was removed, a small 

 sodium flame was placed before the slit of the spectrometer, and the 

 sodium lines impressed on the plate in their proper position, to serve 

 as a reference. For securing photographs of the dispersion resulting 

 from still denser vapour more light was desirable, and a small prism 

 was accordingly substituted for the grating. This prism barely 

 resolved the sodium lines in the negatives, but gave excellent pictures 

 of the spectrum under strong sodium dispersion Figs. 10 and 11 were 

 obtained in this way. In the latter the flutings in the red and green 

 are discernible, though not very sharp. 



As much of the fine detail is always lost in reproduction I have 

 marked with arrows on fig. 11 the extreme points to which the curved 

 branches adjacent to the absorption band can be traced in the original 

 negative. Eye observations enable one to trace them much farther, 

 for the tips are exceedingly faint, and the light is not very actinic. 



In order to secure accurate measurements of the dispersion in different 

 parts of the spectrum I have made use of two different methods. 



The photographs were not very suitable owing to the fact that 

 during the exposure the density of the vapour varied somewhat, pro- 

 ducing a slight blurring of the image. Measurements of the deviation 

 of the continuous spectrum were accordingly made with a filar micro- 

 meter, an illuminated scale giving the wave-lengths at the points where 

 readings were taken. Several minutes were necessary, however, to 

 get anything like a satisfactory set of readings, and changes occurred 

 even during that short period, as was at once apparent on repeating 

 observations. I accordingly adopted the following method. The 

 telescope of the spectrometer was removed and a telescope objective 

 of nearly two metres focus put in its place. 



In the focal plane of this lens a plate of glass was firmly mounted, 

 on which a horizontal diamond scratch had been ruled. This diamond 

 scratch was brought accurately into coincidence with the narrow 

 continuous-spectrum image in its undeviated position. An eye-piece 

 mounted behind the plate enabled the spectrum to be observed. When 

 the dispersion tube was in good condition, and the deviated branches 

 of the spectrum appeared sharp and steady, their positions on the 

 plate were marked with a writing diamond, the line being drawn along 

 the middle of the spectrum, which was only about a millimetre wide on 

 the plate. The dispersion tube was then removed and various metals 

 fed to the arc, the bright lines being marked on the plate with the 



