SECT. XXV. THE DIFFERENTIAL TELESCOPE. 227 



diminishes as the square of the distance increases, the light of 

 Jupiter is proportionally greater than that of the moon, conse- 

 quently Jupiter's atmosphere reflects more light than the dark 

 volcanic soil of the moon ; thus Professor Secchi observes photo- 

 graphy may in time reveal the quality of the materials of which 

 the celestial bodies are formed. 



The effect of the earth's atmosphere on lunar heat is remarkable. 

 Professor Forbes proved that the direct light of the full moon is 

 incapable of raising a thermometer the one three thousandth part 

 of a Centigrade degree, at least in England ; but at an elevation of 

 8870 feet on the Peak of Teneriffe, Mr. Piazzi Smyth found a 

 very sensible heat from the moon, although she was then 19 

 south of the equator ; so it is no doubt absorbed by our atmo- 

 sphere at lower levels. 



. Some exceedingly interesting experiments might be made by 

 means of a telescope having a prism attached to its objective 

 extremity, and furnished with a micrometer, because by it the 

 difference of the illumination of objects might be determined 

 with extreme accuracy as for example, the comparative inten- 

 sity between the bright and dark parts of the moon, the com- 

 parative intensity of the solar light reflected by the moon, and 

 the lumiere cendre, or the light of the earth reflected on the moon, 

 whence a comparison might be made between the light of the sun 

 and that of the earth. Hence also it might be known whether 

 the terrestrial hemispheres successively visible from the moon are 

 more or less luminous, according as they contain more land or 

 water, and at the same time it might be possible to appreciate 

 the more or less cloudy or clear state of our atmosphere, so that 

 in time we might ultimately find in the lumiere cendre' of the 

 moon data upon the mean diaphaneity of different terrestrial 

 hemispheres which are of different temperatures. 



It is found by experience that heat is developed in opaque and 

 translucent substances by their absorption of solar light, but 

 that the sun's rays do not sensibly alter the temperature of 

 perfectly transparent bodies through which they pass. As the 

 temperature of the pellucid planetary space can be but little 

 affected by the passage of the sun's light and heat, neither can 

 it be sensibly raised by the heat now radiated from the earth. 



Doubtless the radiation of all the bodies in the universe main- 

 tains the ethereal medium at a higher temperature than it would 



