284 
DR. J. YOUNG AND PROFESSOR G. FORBES ON THE 
“ After blue equality turned again to red, and B was much brighter ” [than A], “ B 
increasing with increase of speed. Spectrum deflection about | inch.” [This was the 
12th equality.] 
‘'No. 2. Ditto. Same in every way. Blue-red =9 seconds. f inch deflection. 
<c No. 3. Same arrangements. Blue-red =5 seconds nearly. Deflection =|- inch. 
“Want of oil in (1), (2), and (3) made velocity irregular. We tried one more 
observation, but the motion of the wheel seemed quite irregular, and though it gave 
the same value (16 seconds) as No. 1, we could not trust it, because on turning on red 
light again A was brightest. We now oiled the mechanism with great effect in 
steadying the motion and diminishing noise. But the lights had begun to flicker and 
we got no good observations.” 
At 4 p.m. the lights improved. 
“ 4 p.m. —Oiled axles. 
“No. 4. 12th equality. Blue-red =8 seconds. Deflection =- t % inch; but it did 
not look so great a change of colour as usual. 
“No. 5. Same arrangements. Blue-red =10'5 seconds. Deflection =ff inch. 
(The change in colour looked greater.) After equality of blue, tried red again, B was 
far brightest. 
“No., 6. Same arrangements. Blue-reel =6 seconds. Deflection = ff inch. 
“No. 7. 11th equality. Blue-red =7 seconds. Deflection =f inch. 
“No. 8. Ditto. Blue-red = 6*5 seconds. Deflection =f inch.” 
“8.45p.m. —No. 9. Same conditions as No. 1” [12th equality]. “Blue-red 
= 16 seconds. Deflection =-§ inch. 
“No. 10. Same conditions. Blue-red =23 seconds. Deflection = finch. Speed 
varied very slowly. 
“ Oiled the mechanism. 
“No. 11. Blue-red =10 seconds. Deflection =f inch [12th equality]. 
“No. 12. Blue-red = 9*5 seconds. Deflection =f inch [12th equality]. 
“No. 13. Blue-reel =9 seconds. Deflection =f inch [11th equality]. 
“No. 14. Blue-red =7 seconds (good). Deflection =§ inch [llth equality].” 
Deducing these results with the aid of the chronograph tests for the mercury 
method, we obtain the following 14 percentages for the values for 
di fference of velocity of blue and red light ._ 
mean velocity of light 
(1) 3'20 
(4) 1*60 
(7) 1-40 
(10) 4*60 
(13) 1*80 
(2) 1‘80 
(5) 2*10 
(8) 1*30 
(11) 2*00 
(14) 1*40 
(3) 1‘0Q 
(6) 1*20 
(9) 3*20 
(12) D90 
