Fluorescence

Prof. Stokes discovered that certain substances had the power of absorbing invisible rays of high refrangibility and correspondingly short wave-length, and of emitting these rays so far lowered in refrangibility as to be luminous. This phenomenon he called fluorescence. By the application of this principle it has been made po ible to render visible the ultra-violet parts of the solar spectrum, the waves of which under normal conditions are too short to affect the eye. If this invisible part of the spectrum be thrown upon a screen freshly washed with sulphate of quinine, it will become visibly bluish, and if the spectrum be very pure the character and position of the dark lines in this portion may be studied.