Chemical Constitution of Saline Solutions. 269 
One advantage of the 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. Hye observations do not carry 
measurements further at the blue end of the spectrum than ) 4000, but photography 
admits of measurements up to \ 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-leneths 
much greater in the latter than in the former, as the following readings from the 
interpolation curve illustrate :— 
: Different 5 Different 
Measurements, | Corresponding | wy Measurements, | Corresponding 
Colour. | Hundredths of valine: in Jeort elues oe Colour. | Hundredths of ‘Wallees in pe eeue oa 
an inch. Wave-lengths. | 6¢ an inch. an inch. Wave-lengths. | of an inch. 
(B 13-4 6867) (H 151-6 3968) 
Rep. 14:0 6864 VIOLET. 148:0 3991 
70 10 
15:0 6794. 149:0 3981 
48 9 
16:0 6746 150:0 3972 
46 9 
17:0 6700 151:°0 39638 
46 10 
18:0 6654 152:0 3953 
(P 242°6 3359) 
(D 38:0 5894) ULtrRa- 248:0 3335 
VIOLET. 5 
YELLOW. 48:0 5730 249:°0 3330 
30 4 
49:0 5700 250:0 3326 
30 | 5 
50°0 5670 | 251:0 | 3021 
31 | | 4 
51:0 56389 252°0 3327 
32 | | 5 
52°0 5607 253°0 3312 
The value of 1th of an inch in tenth-metres of wave-lengths is at least nine 
times as greatin 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 refrangibility of the rays measured. Absorption spectra, 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 
TRANS. ROY. DUB. SOC., N.S. VOL. VII., PART VIII, 2P 
