VELOCITY OF WHITE AKD OF COLOURED LIGHT. 
283 
“No. 1. Green to orange. . . J-inch, 12'5 seconds, 12th equality, good 
“No. 
2. 
Green to reddish-orange f-inch, 15 
,, 13 th 
„ good 
“ No. 
3. 
Blue to orange . . . y^-inch, 9 
„ 14 th 
,, fair 
“ No. 
4. 
Deep blue to blood-red f-inch, 9 
,, 14 th 
„ very good. 
“ This was a most successful mode of observation. Though B was much greater than 
A the equality was well determined each time.” 
In reducing these observations it is clearly impossible to take into account the exact 
refrarigibilities or wave-lengths of the colours named. The general change was from a 
slightly greenish-blue to a red tinged with orange. Reducing as before, we have— 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
Difference in velocity (blue and red) 
total velocity 
= 1'35 per cent. 
> 5 
3? 
: 1 ’56 
: 0 ’ 9 0 
:0*90 
March 8, 1 p.m.— It might, perhaps, be thought that we had now thoroughly tested 
and confirmed our first conclusions by the variety of our tests. But we were anxious 
to leave no room for doubt and to vary the tests in every possible way, and we thought 
that some advantage might be gained by changing entirely our method of altering 
the speed, so as gradually to increase it, the reverse of what we had done with the 
indiarubber. Accordingly we attached an iron crucible to the weights and led a 
thick indiarubber tube from it up to a funnel with a stop-cock, in reach of the 
observer. We filled this with mercury, and having put on such weights as were 
barely sufficient to produce equality, we turned on red light (using the prism method) 
and opened the stop-cock. So soon as equality of red lights was attained, we said 
“ Stop ” to the assistant, who then commenced to count seconds on the clock. Mean¬ 
time he had instantaneously changed the colour of the light to blue. When the blue 
equality arrived we again said “Stop.” He gave us the interval in seconds, and the 
deflection of the spectrum in fractions of an inch, which we entered in the observing 
book. Chronograph tests were afterwards made to measure the increase in speed 
produced in a second by the flow of mercury. This method seemed to us to be hardly 
of so great delicacy as the indiarubber method, but we felt much interest in seeing 
whether so entirely a different method could give results approximating to those 
obtained in the previous researches. 
The following remarks are extracted from the observing book :— 
“March 8, 1 p.m.— No. 1 . Blue-red =16 seconds. Exact time uncertain to a few 
seconds. General effect positive ” the blue equality required a higher speed]. 
2 o 2 
