282 
DR J. YOUNG AND PROFESSOR G. FORBES ON THE 
of the 10th and 11th equalities. The reductions are in the hands of the Royal Society, 
and they gave us— 
10th equality . . 0*34 revolutions per second diminution in 1 second. 
11th „ . . 0-29 
9th „ . . 0'31 
Reducing the first and second observations as before, allowing for a diminution of 
speed of 0*49 revolution a second, per second— 
*49 v 11 y 100 
From No. 1 we have - tux - =1*28 percent. 
No. 2 
420 
*49X11-5X100 
460 
= 1*22 
Chronograph records were afterwards taken at about the speeds of the 9th, 10th, and 
11th equalities, and the speeds read off at intervals of about 8 or 10 seconds. Thus 
we obtained the percentage loss of speed hi one second. Reducing the observations 
of this day we have— 
No. 3. 9th equality; 
No. 4. 10th 
No. 5. 9th „ 
No 6. 10th „ 
No. 7. 10th „ 
No. 8. 11th ,, 
difference of velocity (blue and red) 
total velocity 
99 
99 
= 1 ‘71 per cent. 
= 1*55 „ 
99 
99 
99 
99 
9 9 
99 
= 3-14 
= 1T0 
= 1*40 
99 
99 
99 
99 
99 
= 0-68 
99 
Mean of the days observations=l*51 
March 1, 1881.—In the interval between February 27 and March 1 we removed the 
wedge under the toothed wheel and brought the mechanism into an upright position. 
Seeing that the solutions which we had hitherto used to produce our colours allowed 
colours of very different refrangibilities to pass, we thought it would be satisfactory to 
try the effect of pure prismatic colours. To do this we mounted a bisulphide of carbon 
bottle-prism in front of the condenser and succeeded in throwing a pure spectrum upon 
the toothed wheel. The distance traversed by the rays from the prism to the toothed 
wheel was 24 \ inches. The electric lamp was mounted upon rollers so that we could 
easily move it to change the colours. A motion of only about ^-inch of the spectrum 
changed the colour of the stars from blue to red. This was a very convenient and 
pleasant method of working. The indiarubber was used as before. The following 
remarks are extracted from the observing book;— 
“ Reduced velocity by stretched indiarubber. Used prism for colours. Deflection 
of spectrum in inches given below [this was measured approximately by our assistant]. 
