286 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
March 8.—Spectrum used for colour. Mercury used to increase speed gradually. 
Fourteen obser nations— 
(1) 3-20 (4) 1*60 
(2) 1*80 (5) 2G0 
(3) 1-00 (6) 1-20 
(7) 1-40 (10) 4-60 
(8) 1-30 (11) 2‘00 
(9) 3'20 (12) 1-90 
Mean percentage = 2'03. 
(13) 1*80 
(14) 1-40 
Tabular statement of percentage differences in tire velocity of red and of blue light. 
February 21. 
February 23. 
February 24. 
February 25. 
February 27. 
March 1. 
March 8. 
April 26. 
April 27. 
+ 3-36 
4-0*29 
4-1*28 
4-1*35 
4-3*20 
1T4 
0*43 
1*22 
1*56 
1*80 
2-40 
0*70 
1*71 
0*90 
1*00 
2-88 
hj 
o 
1*55 
0*90 
1*60 
5-40 
3*14 
2*10 
hj 
o 
ce 
M • 
o 
2-46 
03 
i— 1 • 
1*10 
1*20 
03 
cf- 
2-40 
< 
1*40 
1*40 
H- » 
< 
2*52 
cd 
cd 
0*68 
1*30 
CD 
CD 
CD 
CD 
tts 
CD 
3*20 
ft 
CD* 
ft 
or 
c 
r-t- 
4*60 
O 
e+- 
o 
r+- 
2*00 
1*90 
1*80 
1*40 
Mean + 2*82 
+ . • • 
-... 
4-0*47 
4-1*51 
4-1*17 
+ 203 
+ . . . 
+ . . . 
General mean of 37 observations =1*8. 
We cannot account for the apparently negative effect obtained on February 24. 
But when we consider, that it was only for a short time that the observations were 
attempted, and that the appearance of a negative effect was extremely feeble (almost 
doubtful), and that in so delicate an observation the state of our health might affect 
the sensitiveness of our appreciation of colour effects, and that on other days the 
positive effect was most marked and indubitable, as recorded in numerous passages in 
our observing book, and emoted above,—when we consider all these things we cannot 
place this single dubious night's evidence in opposition to the overwhelming testimony 
of so many positive observations. It was too much to expect that we should be able 
in the first instance to measure, by the differential methods described above, the 
difference produced by a given difference of wave-length with very great precision. 
But there seems little room for doubt that the general mean of these 37 observations 
cannot be far from the truth, and we may affirm that when the wave-length changes 
from about \=50 tenth-metres to about \=6 0 tenth-metres, the velocity changes 
about 1‘8 per cent., or in any case somewhat over 1 per cent. 
