very of the cause of this
wonder.'[21]

Goethe in his 'Farbenlehre' thus describes the fluorescence of
horse-chestnut bark:--'Let a strip of fresh horse-chestnut bark be
taken and clipped into a glass of water; the most perfect sky-blue
will be immediately produced.'[22] Sir John Herschel first noticed and
described the fluorescence of the sulphate of quinine, and showed that
the light proceeded from a thin stratum of the solution adjacent to
the surface where the light enters it. He showed, moreover, that the
incident beam, although not sensibly weakened in luminous intensity,
lost, in its transmission through the solution of sulphate of quinine,
the power of producing the blue fluorescent light. Sir David Brewster
also worked at the subject; but to Professor Stokes we are indebted
not only for its expansion, but for its full and final explanation.

Sec. 3. _The Heat of the Electric Beam. Ignition through a Lens of Ice.
Possible Cometary Temperature_.

But the waves from our incandescent carbon-points appeal to another
sense than that of vision. They not only produce light, but heat, as a
sensation. The magnified image of the carbon-points is now upon the
screen; and with a suitable instrument the heating power of the rays
which form that image might be readily demonstrated. In this case,
however, the heat is spread over too large an area to be very intense.
Drawing out the camera lens, and causing a movable screen to approach
the lamp, the image is seen to become smaller and smaller; the rays at
the same time becoming more and more concentrated, until finally they
are able to pierce black paper with a burning ring. Pushing back the
lens so as to render the rays parallel, and receiving them upon a
concave mirror, they are brought to a focus; paper placed at that
focus is caused to smoke and burn. Heat of this intensity may be
obtained with our ordinary camera and lens, and a concave mirror of
very moderate power.

[Illustration: Fig. 48.]

We will now adopt stronger measures with the radiation. In this larger
camera of blackened tin is placed a lamp, in all particulars similar
to those already employed. But instead of gathering up the rays from
the carbon-points by a condensing lens, we gather them up by a concave
mirror (_m_ _m'_, fig. 48), silvered in front and placed behind the
carbons (P). By this mirror we can cause the rays to issue through the
orifice in front of the camera, either parallel or convergent