e red end of
the spectrum they are very much crowded, while towards the blue end they
are more dispersed. Hence, if one were measuring the heating power of
such a spectrum, many more rays would fall upon an equal surface of the
thermo-pile at the red end than at the blue end; therefore the
indications of the galvanometer would be fallacious. Before any thing
definite could be known about the matter, it would plainly be necessary
to work with an equal dispersion of all the rays. This was effected a
few years ago by Dr. Draper of New York. He took the spectrum produced
by diffraction instead of refraction, and measured that. In that way it
was found that the heating power of the spectrum is equal in every part
of it; and hence the pictures in treatises on physics that represent the
heating power of the spectrum to be concentrated at the red end is not
true save where the spectrum is irregularly produced. As for vision, the
mechanical structure of the eye is such that radiant vibrations having a
wave length between 1/37000 in. and 1/60000 in. can affect it, while
longer or shorter wave lengths can not. Such waves we call light, but it
is not at all improbable that some animals and insects have eyes adapted
to either longer or shorter wave-lengths; in which case, what would be
perfectly dark to us would be light to them. It is a familiar enough
fact, that many animals, such as dogs, cats, rats, and mice, can see in
the night. Some horses may be trusted to keep in the road in a dark
night, when the driver cannot see even the horse itself. This has
usually been accounted for by saying that their eyes are constructed so
as to collect a greater number of luminous rays. It is much better
explained by supposing their eyes to be constructed to respond to
wave-lengths either greater or less than those of mankind.

A ray of light, then, consists of a single line of undulations of a
definite wave length, such that if it falls upon the eye it will produce
sight; if it falls upon a thermo-pile it heats it by just the same
quantity that another wave-length would heat it; if it falls upon matter
in unstable chemical relations, it will do chemical work, depending upon
the kinds of matter. A red ray is as effective for some substances as a
violet ray is for others. The statement, then, so often lately made to
do certain analogical work, namely, that a ray of light consists of
three distinct parts, which may be separated from each other, and