MOONLIGHT. 137 



substance, the measure of the other's degrees. The 

 variations of light are so very delicate in themselves, 

 and they are so much confused by the variations of 

 colour, that it is scarcely possible to obtain any con- 

 trivance by which light can be made the measure 

 even of itself. 



Various instruments called photometers, that is, 

 '' light measures," have been invented by ingenious 

 men ; but the majority, if not the whole of these, 

 are affected by, and therefore measure heat, and not 

 light ; and thus they are, in truth, thermometers, or 

 heat measures of more nice construction and greater 

 sensibility than the common ones. 



It is, indeed, exceedingly difficult even to contem- 

 plate light without having the notion of heat along 

 with it ; and, indeed, we have not much knowledge of 

 especially great degrees of heat, without light along 

 with it. In poetical language it is not uncommon 

 to speak of " the wan cold moon," and " the cold 

 moonbeams ;" and there is truth as well as poetry 

 in those expressions. It has been mentioned that 

 the red rays of the sun penetrate the most readily 

 into the substances on which they fall ; and the 

 greatest heat, which is at the red end of the spec- 

 trum, penetrates still more readily than the red rays. 

 Now, our moonlight really comes from the sun, and 

 is reflected to us from the surface of the moon, just 

 as we can throw light into a dark room by a mirror, 

 or by whitewashing a wall opposite the door on 

 which light can fall. Now the heat of the sun's 

 light, and also the greater part of the red rays, enter 

 into and are absorbed by the moon ; and thus moon- 

 light wants the golden brightness of the direct rays 

 of the sun, and is in consequence silvery, and has a 

 little of a bluish tint in it. 



This " soft moonlight," not only delightfully varies 



the months with its waxing, its fulness, its waning, 



and its extinction, and not only gives us landscapes 



of new and softened tone, which it would be alto- 



M 2 



