1868. | Luminous Intensity of Light. 359 
until the two halves of the paper are equally illuminated. In 
Bunsen’s photometer, which is the one now generally used, the lights 
shine on opposite sides of a dise of white paper, part of which has 
been smeared with melted spermaceti to make it more transparent. 
When illuminated by a front light the greased portion of the paper 
will look dark; but if the observer goes to the other side of the 
paper, the greased part looks the lighter. If therefore lights of 
unequal intensity are placed on opposite sides of a piece of paper so 
prepared, a difference will be observed ; but by moving one backwards 
or forwards, so as to equalize the intensity, the whole surface of the 
paper will appear uniformly illuminated on both sides. This photo- 
meter has been modified by many observers. By some the disc of 
paper is moved, the lights remaining stationary ; by others the whole 
is enclosed in a box, and various contrivances are adopted to increase 
the sensitiveness of the eye and to facilitate calculation: but in all 
these the sensitiveness is not materially augmented, as the eye 
cannot judge of very minute differences of illumination approxi- 
mating to equality. 
In 1833 Arago described a photometer in which the pheno- 
mena of polarized light were employed. This instrument is fully 
described, with drawings, in the tenth volume of the Guvres Com- 
pletes de Francois Arago; but the description, although voluminous, 
is far from clear. The principle of its construction is founded on 
“the law of the square of the cosines,” according to which polarized 
rays pags from the ordinary to the extraordimary image. - The 
knowledge of this law, he says, will not only prove theoretically 
important, but will further lead to the solution of a great number 
of very important astronomical questions. Suppose, for example, 
that it is wished to compare the luminous intensity of that portion 
of the moon directly illuminated by the solar rays with that of the 
part which receives only light reflected from the earth, called 
the partie cendrée. Were the law in question known, the way 
to proceed would be as follows:—after having polarized the moon's 
light, pass it through a doubly refracting crystal, so disposed 
that the rays, not being able to bifurcate, may entirely undergo 
ordinary refraction. A lens placed behind this crystal will there- 
fore show but one image of our satellite; but as the crystal in 
rotating on its axis passes from its original position, the second 
image will appear, and its intensity will go on augmenting. The 
movement of the crystal must be arrested at the moment when, in 
this growing extraordinary image, the segment corresponding to 
the part of the moon illuminated by the sun, exhibits the intensity 
of the ashy part shown by the ordinary image. From these data it 
is easy to perceive, he says, that the problem is capable of solution. 
In another part of the same volume, after speaking of the 
polariscope which goes by his name, Arago writes: —“I have now 
VOL. V. 2c 
