254 



THE MAJOR PLANETS. 



such circumstances must really be. The light of the full moon is about three 

 hundred thousand times less than that of the sun ; consequently it follows that 

 the light of day at Herschel will be equal to the light of more than one thou- 

 sand full moons. 



Independent of this consideration, however it will be remembered, as we 



have urged on another occasion, that the perception of the brightness of light, 



does not depend only upon the density of the light itself; but also, upon the 



magnitude of the pupil of the eye and the sensibility of the retina. Nothing 



^ can be more easy to imagine than a very small alteration of the proportions of 



? the eye, without even the necessity of admitting any in its structure, which 



7 would render the light of the sun at Herschel as efficient for the purpose of 



> vision as at the earth. 



It has been, in various popular works, and even in some strictly scientific 

 treatises, urged that the cold which prevails at this and other remote planets, 

 must be so intense that the liquids of our globe could not exist there ; and, on 

 the other hand, that at the piauet Mercury, a degree of heat must exist equally 



