3 20 POPULAR LECTURES AND ADDRESSES. 



the eye, our knowledge of vibrations at the other 

 end of the scale only comprehends those having 

 twice the frequency of violet light. In round 

 numbers 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 1,600 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 



