all events a portion of the time allotted to me in answering
the question why the sun appears red in a fog. I must first of all appeal
to what every one who frequents this theater is so accustomed, viz., the
spectrum. I am going not to put it in the large and splendid stripe of
the most gorgeous colors before you, with which you are so well
acquainted, but my spectrum will take a more modest form of purer colors,
some twelve inches in length.

I would ask you to notice which color is most luminous. I think that no
one will dispute that in the yellow we have the most intense luminosity,
and that it fades gradually in the red on the one side and in the violet
on the other. This, then, may be called a qualitative estimate of
relative brightnesses; but I wish now to introduce to you what was novel
last year, a quantitative method of measuring the brightness of any part.

Before doing this I must show you the diagram of the apparatus which I
shall employ in some of my experiments.

[Illustration: FIG. 1.--COLOR PHOTOMETER.]

RR are rays (Fig. I) coming from the arc light, or, if we were using
sunlight, from a heliostat, and a solar image is formed by a lens, L_{1},
on the slit, S_{1} of the collimator, C. The parallel rays produced by
the lens, L_{2}, are partially refracted and partially reflected. The
former pass through the prisms, P_{1}P_{2}, and are focused to form a
spectrum by a lens, L_{3}, on D, a movable ground glass screen. The rays
are collected by a lens, L_{4}, tilted at an angle as shown, to form a
white image of the near surface of the second prism on F.

Passing a card with a narrow slit, S_{2}, cut in it in front of the
spectrum, any color which I may require can be isolated. The consequence
is that, instead of the white patch upon the screen, I have a colored
patch, the color of which I can alter to any hue lying between the red
and the violet. Thus, then, we are able to get a real patch of very
approximately homogeneous light to work with, and it is with these
patches of color that I shall have to deal. Is there any way of measuring
the brightness of these patches? was a question asked by General Festing
and myself. After trying various plans, we hit upon the method I shall
now show you, and if any one works with it he must become fascinated with
it on account of its almost childish simplicity--a simplicity, I may
remark, which it took us some months to find out. Placing a rod before
the screen, it ca