Page:On Radiation.djvu/18

 rays of high heating power, but unsuited to the purposes of vision; 2° Of luminous rays which display the following succession of colours, red, orange, yellow, green, blue, indigo, violet; 3° Of ultra-violet rays which, like the ultra-red ones, are incompetent to excite vision, but unlike them possess a very feeble heating power. In consequence, however, of their chemical energy these ultra-violet rays are of the utmost importance to the organic world.

2. Origin and Character of Radiation. The Ether.

When we see a platinum wire raised gradually to a white heat, and emitting in succession all the colours of the spectrum, we are simply conscious of a series of changes in the condition of our eyes. We do not see the actions in which these successive colours originate, but the mind irresistibly infers that the appearance of the colours corresponds to certain contemporaneous changes in the wire. What is the nature of these changes? In virtue of what condition does the wire radiate at all? We must now look from the wire as a whole to its constituent atoms. Could we see those atoms, even before the