Chemical Constitution of Saline Solutions. 



269 



One advantage of tlie photographic method is seen in the registration of the 

 extent to which the ultra-violet rays are absorbed. Differences of exposure with 

 the same photographic plates do not materially alter the character of the absorp- 

 tion bands, or the extent of the transmitted rays, but differences in the tempera- 

 ture of the solution do make a marked difference. Eye observations do not carry 

 measurements further at the blue end of the spectrum than X 4000, but photography 

 admits of measurements up to X 2872. 



The small dispersion in the red compared with the violet and ultra-violet 

 renders the value of linear measurements expressed in terms of wave-lengths 

 much greater in the latter than in the former, as the following readings from the 

 interpolation curve illustrate : — 



The value of y^-oth of an inch in tenth-metres of wave-lengths is at least nine 

 times as great in the red as in the ultra-violet, six times as great in the yellow, and 

 twice as great in the violet. There is a limit to the accuracy with which absorp- 

 tion spectra may be measured ; this differs with different spectra and in the same 

 spectrum with the refraugibility of the rays measured. Absorption sjDectra, as a 

 rule in the ultra-violet, are much sharper than those in the visible spectrum ; it is 

 thus easy to measure them to four figures, but in the red, yellow and green, it is 



TKANS. EOY. DUB. SOC, N.S. VOL. VII., PAET VUI. 2 P 



