88 
Mr. J. F. W. Herschel on the action of 
can be examined with any certainty. The combination I 
employed for the extreme red was such, that when the 
whole spectrum thrown on a white screen was viewed through 
it, it was seen reduced to a perfectly circular, well defined, 
deep red image, whose centre fell on the very farthest termi- 
nation of the red as seen by the naked eye, and whose cir- 
cumference attained, or perhaps surpassed the point where 
the maximum of the calorific rays has been supposed to be 
situated. In like manner, when the same spectrum was ex- 
amined with the violet combination, a very slightly elongated 
violet image became perceptible, but every trace of the indigo, 
and the brighter portion of the violet rays was extinguished. 
For observations on the indigo, and all the more refrangible 
portion, I employed similar artifices, without which I found 
it perfectly impracticable to obtain any regular and com- 
parable results. 
1 i jr 
The coefficient — — in our formula being the only part not 
immediately deduced from observation, it is evident that the 
assumption -j = ~ r must be widely erroneous in the present 
instance, and it therefore becomes necessary to ascertain the 
values of l by direct measures. This is rendered easy by 
the equation ( e ) which gives 
r . sin 6 . sin S' 
L t • • • 
ft. cos f 
We have only therefore to observe the inclinations of a plate 
of known thickness, properly cut and adjusted to 45 0 azimuth, 
which correspond to the alternate disappearances of the or- 
dinary and extraordinary images, at which points the values 
of n are -f, ^ c; computing then the values of 0, 6', 
and (p, and substituting, we get the values of /, without 
