GENERAL ACCOUNT OF RADIATION. 225 



but when they are absorbed by lampblack, say, they all become merely 

 heat, of the same quality after absorption, whether absorbed red or 

 absorbed yellow. The absence of heating effect at the blue end is merely 

 due to want of sensitiveness of the heat detector, the eye being far more 

 easily excited than the thermopile. 



But this does not tell us anything about the radiation received from 

 dark bodies. We may easily, however, explain the nature of this dark 

 radiation in answering a question which naturally arises. Does this 

 narrow range in the wave-length of the visible spectrum correspond to 

 limits existing in the external disturbance, or merely to limits in our 

 light-sense perception? 



Suppose that we replace the eye, as the recording instrument, by a 

 thermopile deflecting a galvanometer-needle. Placing this in various 

 parts of the visible spectrum, we get, as we have seen, deflections hardly 

 visible in the blue and green, but rising towards the red, showing that 

 the light is converted into heat in the thermopile. Now, taking the 

 thermopile out beyond the red, it still gives indications which show 

 clearly that longer wave-radiations are falling on the screen there, and 

 these are termed "infra "-red radiations But just as a coloured glass 

 absorbs light of a certain wave-length in the visible spectrum, while 

 transmitting that of other lengths, so what we call transparent glass 

 absorbs some of the radiation of long wave-lengths, though transmitting 

 that of the visible spectrum. The effect beyond the red is, therefore, dimin- 

 ished by absorption in the glass lens and prism. If we use rock-salt 

 instead of glass for prism and lenses the absorption is much more limited, 

 and the effect on the thermopile is greater On moving the thermopile 

 to the ultra-violet, we find no sensible effect, and we must use a more 

 delicate test for the presence of the energy. If we paint a sheet of 

 paper with a solution of sulphate of quinine in dilute acid, and expose it 

 in the ultra-violet region, there is an action on the solution, and it gives 

 out visible radiation. Or, there are ultra-violet short wave-length radi- 

 ations, which are absorbed by sulphate of quinine, and given out again 

 by it as radiations of wave-length long enough to excite the retina. Or 

 still more marked is the effect on an interposed screen of the kind used 

 for the detection of X-rays, brilliant bands of green light appearing in 

 the region beyond the violet. These ultra-violet radiations are especially 

 active in exciting the chemical changes employed in ordinary photography. 

 That these extreme radiations are also of wave form, has been shown 

 from the fact that they exhibit the phenomena of interference and of 

 polarisation, which we can explain only by the wave theory. 



We conclude then, that radiant energy is, in form, a wave dis- 

 turbance, the lengths of the waves extending over a wide range, but to 

 one particular part of the range the retina is sensitive, and we call the 

 energy within this range light. This is probably the part of the energy 

 which is present in greatest quantity in sunlight. As we see most things 

 by reflected sunlight, we see them more easily by being especially sensitive 

 to that kind of radiation which is present in greatest quantity. 



Radiometers only Measure Energy Streams and do not 

 Indicate Quality. The various radiometers, unlike the eye, take no 

 account of the wave-length of the stream of radiation which they mea- 

 sure. Their indications are proportional to the rate at which they are 



P 



