towards the detection of the nature of colours.  Take Lignum, Nephriticum, and with a knife cut it into thin slices:  put about a handful of these slices into two or three or four pounds of the purest spring water.  Decant this impregnated water into a glass phial; and if you hold it directly between the light and your eye, you shall see it wholly tinted with an almost golden colour.  But if you hold this phial from the light, so that your eye be placed betwixt the window and the phial, the liquid will appear of a deep and lovely ceruleous colour.’

‘These,’ he continues, ’and other phenomena which I have observed in this delightful experiment, divers of my friends have looked upon, not without some wonder; and I remember an excellent oculist, finding by accident in a friend’s chamber a phial full of this liquor, which I had given that friend, and having never heard anything of the experiment, nor having anybody near him who could tell him what this strange liquor might be, was a great while apprehensive, as he presently afterwards told me, that some strange new distemper was invading his eyes.  And I confess that the unusualness of the phenomenon made me very solicitous to find out the cause of this experiment; and though I am far from pretending to have found it, yet my enquiries have, I suppose, enabled me to give such hints as may lead your greater sagacity to the discovery 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.