364 On the Measwrement of the [ July, 
through the eye-piece two bright discs will be seen, probably of 
different colours. Supposing E represents the standard flame and 
F the light to be compared with it, the latter must now be slid 
along the scale until the two discs of light, seen through the eye- 
piece, are about equal in tint. It has been found most convenient 
not to attempt to get absolute equality in this manner, but to move 
the flame to the nearest inch on one side or the other of equality. 
The final adjustment is now effected at the eye-end, by turning the 
polarimeter one way or the other up to 45° until the images are 
seen without any trace of colour. This will be found more accurate 
than the plan of relying entirely on the alteration of the distance of 
the flame along the scale; and by a series of experimental adjust- 
ments, the value of every angle through which the bundle of plates 
is rotated can be ascertained once for all, when the future calculations 
will present no difficulty. Squaring the number of inches between 
the flames and the centre will give their approximate ratios; and 
the number of degrees the eye-piece rotates will give the number to 
be added or subtracted in order to obtain the necessary accuracy. 
The delicacy of the instrument is very great. With two lamps, 
each about twenty-four inches from the centre, it is easy to distin- 
cuish a movement of one of them to the extent of jth of an 
inch to or fro; and by using the polarimeter an accuracy consider- 
ably exceeding that can be attained. 
The employment of a photometer of this kind enables us to com- 
pare lights of different colours with one another, and leads to the 
solution of a problem which, from the nature of their construction, 
would be beyond the powers of the instruments in general use. So 
long as the observer, by the eye alone, has to compare the relative 
intensities of two surfaces respectively illuminated by the lights 
under trial, it is evident that unless they are of the same tint it is 
impossible to obtain that absolute equality of illumination in the 
instrument which is requisite for a comparison. By the unaided 
eye one cannot tell which is the brighter half of a paper dise 
illuminated on one side with a reddish and on the other with a yel- 
lowish light ; but by using the above-described photometer the pro- 
blem becomes practicable. For instance, on reference to Fig. 1, 
suppose the disc D were illuminated with light of a reddish colour 
and the disc C with greenish light, the polarized discs d' d would 
be reddish and the discs ¢ ¢’ greenish, the central disc ¢ d being of 
the tint formed by the union of the two shades. The analyzing 
prism K and the selenite disc T will detect free polarization in the 
disc ed if it be coloured as readily as if it were white; the only 
difference being that the two discs of light g 7 cannot be brought 
to a uniform white colour when the lights from D and C are equal 
in intensity, but will assume a tint similar to that of ed. When 
the contrasts of colour between D and C are very strong—when, 
