1868. | Luminous Intensity of Light. 361 
a similar white disc likewise illuminated. It is required to com- 
pare the photometric intensities of D and C. (It is necessary 
that neither D nor C should contain any polarized light, but that the 
light coming from them, represented on each disc by the two lines 
at right angles to each other, forming a cross, should be entirely 
unpolarized.) Let H represent a double refracting achromatic 
prism of Iceland spar; this will resolve the disc D into two discs 
d and d’, polarized in opposite directions ; the plane of d being, we 
will assume, vertical, and that of d' horizontal. The prism H will 
likewise give two images of the disc C; the image c being polar- 
ized horizontally, and ¢' vertically. The size of the discs D, C, and 
the separating power of the prism H are to be so arranged that 
the vertically polarized image d, and the horizontally polarized 
image ¢, exactly overlap each other, forming, as shown in the figure, 
one compound dise ¢ d, built up of half the light from D and half 
that from C. 
The measure of the amount of free polarization present in the 
dise ¢ d, will give the relative photometric intensities of D and C. 
The letter I represents a diaphragm with a circular hole in the 
centre, just large enough to allow the compound disc ¢ d to be 
seen, but cutting off from view the side discs c’, d’. In front of 
the aperture in I is placed a piece of selenite of appropriate thick- 
ness for it to give a strongly-contrasting red and green image 
under the influence of polarized light. K is a doubly refracting 
prism, similar in all respects to H, placed at such a distance from 
the aperture in I that the two discs into which I appears to be 
split up are separated from each other, as at g, 7. If the disc ¢c d 
contains no polarized light, the images g 7 will be white, consisting 
of oppositely polarized rays of white light; but if there is a trace of 
polarized light in ¢ d, the two discs g r will be coloured compli- 
mentarily ; the contrast between the green and red being stronger 
in proportion to the quantity of polarized light in ¢ d. 
The action of this arrangement will be readily evident. Let it 
be supposed in the first place that the two sources of light, D and C, 
are exactly equal. They will each be divided by H into two discs, 
d' d and ¢ c¢’, and the two polarized rays of which ¢ d is com- 
pounded will also be absolutely equal in intensity, and will neutralize 
each other and form common light, no trace of free polarization 
being present. In this case the two discs of light g 7 will be 
colourless. Let it now be supposed that one source of light (D for 
instance) is stronger than the other (C). It follows that the two 
images d’ d will be more luminous than the two images ¢ c’, and that 
the vertically polarized ray d will be stronger than the horizontally 
polarized ray c. The compound disc ¢ d will therefore shine with 
partially polarized light, the amount of polarization being in exact 
ratio with the photometric intensity of D over C. a ne ase the 
c 
