SOLAR LIGHT AND HEAT. 



The relative apparent magnitudes are exhibited in fig. 6, at 

 E and IS T . 



It would, however, be a great mistake to infer that the light of 

 the sun at Neptune approaches in any degree to the faintness of 

 that of Yenus at the earth. If Yenus, when that planet appears 

 as a morning or evening star, with the apparent diameter of 60", 

 had a full disc (instead of one halved or nearly so, like the moon 

 at the quarters), and if the actual intensity of light on its surface 

 were equal to that on the surface of the sun, the light of the 



Fig. 6. 



planet would be exactly that of the sun' at Neptune. But the 

 intensity of the light which falls on Yenus is less than the inten- 

 sity of the light on the sun's surface in the ratio of the square of 

 Yenus' distance to that of the sun's semidiameter, upon the 

 supposition that the light is propagated according to the same law 

 as if it issued from the sun's centre ; that is, as the square of 37 

 millions to the square of half a million nearly, or as 37* : |, that 

 is, as 5476 to 1. If, therefore, the surface of Yenue reflected 

 (which it does not) all the light incident upon it, its apparent 

 light at the earth (considering that little more than half its 

 illuminated surface is seen) is about 11000 times less than the 

 light of the sun at Neptune. 



Small, therefore, as is the apparent magnitude of the sun at 

 Neptune, the intensity of its daylight is probably not less than 

 that which would be produced by about 20000 stars shining at 

 once in the firmament, each being equal in splendour to Yenus 

 when that planet is brightest. 



In addition to these considerations, it must not be forgotten 

 that all such estimates of the comparative efficiency of the illu- 



191 



