10 Mr Barlow On the Practical Construction 
purpose a space is left open above the prism in the face of the 
diaphram. This being done, make it fast in this position by 
the set-screw. Remove the plumb-line, and looking at the 
card-paper strip, its upper edge will be seen strongly tinged with 
violet and blue, and the lower edge with red and yellow. Now, 
put on the tube and prism, Fig. 9., placing the base of this 
prism upwards and horizontal, and then, on examining the strip 
of card-paper again (the latter prism being the stronger of the 
two in producing dispersion), the upper edge will be found 
tinged with red and yellow, and the lower with the violet. If 
now, Fig. 9. be gradually turned round either to the right 
or left, while the eye is still regarding the card-paper strip, 
the colours on both edges of the paper will diminish, and, at 
length, in a certain position, will wholly disappear. This being 
well and carefully observed, register the reading shown by the 
vernier on the graduated circle above mentioned. Then turn 
the circle back in the other direction till the colours again dis- 
appear, and again register the reading shewn by the vernier : 
call half the intercepted arc between the two readings M. (This 
will be the difference of the readings, if both are on the same 
side of zero, but the sum if on different sides). 
Let this observation, which is very simple, be repeated several 
times, and the mean of all the results taken for the value of M. 
11. Computation for the Ratio of Dispersion. 
1. Let the prism fixed in Fig. 8. or the fixed prism, and which 
we here suppose to be the plate glass, be called A, and let 
this letter also denote its angle; and let the flint prism in 
Fig. 9. be called B, which may also denote its angle. 
Then, 
To the log sine of angle A, add the log of its index of re- 
fraction ; and from the sum substract the log of the in- 
dex of the refraction of B, and find the angle, of which 
the remainder is the log sine, and call it angle a. 
3. To the log tangent of angle B, add the log cosine of angle 
M, and find the angle of which the sum is the log tan- 
gent, and call it angle b. 
4. From a subtract b, and call the remainder = c. 
