THE WAVE THEORY OF LIGHT 273 



having twice the frequency of violet light. In round num- 

 bers we have 4 octaves of light, corresponding to 4 octaves 

 of sound in music. In music the octave has a range to a 

 note of double frequency. In light we have one octave of 

 visible light, one octave above the visible range and two 

 octaves below the visible range. We have 100 per second, 

 200 per second, 400 per second (million million understood) 

 for invisible radiant heat; 800 per second for visible light, 

 and i, 600 per second for invisible or actinic light. 



One thing common to the whole is the heat effect. It is 

 extremely small in moonlight, so small that until recently 

 nobody knew there was any heat in the moon's rays. 

 Herschel thought it was perceptible in our atmosphere by 

 noticing that it dissolved away very light clouds, an effect 

 which seemed to show in full moonlight more than when we 

 have less than full moon. Herschel, however, pointed this 

 out as doubtful; but now, instead of its being a doubtful 

 question, we have Professor Langley giving as a fact that 

 the light from the moon drives the indicator of his sensi- 

 tive instrument clear across the scale, showing a compara- 

 tively prodigious heating effect ! 



I must tell you that if any of you want to experiment with 

 the heat of the moonlight, you must measure the heat by 

 means of apparatus which comes within the influence of the 

 moon's rays only. This is a very necessary precaution; if, 

 for instance, you should take your Bolometer or other heat 

 detector from a comparatively warm room into the night 

 air, you would obtain an indication of a fall in temperature 

 owing to this change. You must be sure that your apparatus 

 is in thermal equilibrium with the surrounding air, then 

 take your burning-glass, and first point it to the moon and 

 then to space in the sky beside the moon; you thus get a 

 differential measurement in which you compare the radiation 

 of the moon with the radiation of the sky. You will then 

 see that the moon has a distinctly heating effect. 



To continue our study of visible light, that is undulations 

 extending from red to violet in the spectrum (which I am 

 going to show you presently), I would first point out on this 

 chart (Fie. 120) that in the section from letter A to letter 

 D we have visual effect and heating effect only ; but no ordi- 



