278 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
a few trials the results were wonderfully accordant. The following extract from the 
observing book contains all the observations made upon that day :— 
41 February 21.—No. 1. At 5.30 p.m. Two lead weights and a. Blue to red—interval 
= 28 seconds.” 
The following were between 7 and 8 p.m. :— 
44 No. 2. 
Two lead 
weights and a + hook. 
Interval = 9^ seconds. 
44 No. 3. 
Weights 
as before 
• • • » 
Interval =20 seconds. 
44 No. 4. 
55 
55 
» • « i 
Interval =24 seconds. 
“ No. 5. 
55 
55 
+ 6. 
Interval =45 seconds. 
44 No. 6. 
55 
55 
» i « * 
Interval = 20^ seconds. Very good. 
44 No. 7. 
55 
55 
. . . 
Interval =20 seconds. 
44 No. 8. 
55 
55 
Interval =21 seconds.” 
The following remark is added : — 
44 During the whole of to-day—while trying different methods to detect the differ¬ 
ence in velocity of different coloured rays—I have at every step been struck by the 
enormous difference that exists. It is easy to get a velocity for which A is greater 
for blue light while B is greater for red light. Sometimes, when A and B are equal 
for blue, A or B is almost invisible for red, and the other at [near ?] its maximum.” 
On the evening of February 21 chronograph measurements were made to determine 
the loss of speed after tile indiarubber had been in action for some definite interval of 
time. The interval adopted was about 18 seconds. The observations and reductions 
are in the hands of the Boyal Society. The results of four such determinations are as 
follows :— 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
Revolutions per second — 
At beginning of observation . . 
At end „ „ • • 
487*"22 
480 r- 28 
514'-77 
499''64 
502'"33 
49P-45 
502'-3 3 
496 r, 60 
Diminution of speed in 18 seconds . . 
i5 „ 1 second . 
1 
O'-94 
0-38 
13'T3 
O'-73 
HF-88 
0'-60 
4'-73 
O'-2 6 
Average .... =0'49 revolution per second. 
Although these measurements vary a little amongst each other, they give us 
sufficiently well a rough knowledge of the rate at which the indiarubber reduces the 
speed. 
The average interval of time between the equality of red and blue lights was 23 - 5 
seconds. This multiplied by 0'49 gives us a difference of 11 ‘5 revolutions a second, 
