NATURAL PHILOSOPHY. 139 



THE EXTINCTION OF LIGHT IN THE ATMOSPHERE. 



IN a letter from Madras, Capt. Jacob refers to Prof. Forbes's inves- 

 tigations upon the extinction of light in the atmosphere, and adds, 

 " On commencing work with heliotropes in 1837, I soon found that 

 for long distances it was necessary to enlarge the apertures more than 

 in the simple ratio of the distance ; and before the end of the first sea- 

 son I had formed a scale of apertures for corresponding distances, 

 which, when finally corrected, stood as follows: 



Aperture. Maximum Distance. Maximum Distance, 



Inches. Miles. without absorption. 



0.5 15 15 



10 23 30 



2.0 33 60 



4.0 45 120 



8.0 60 240 



" At these distances the light was just visible to the naked eye in 

 clear weather, and when seen over a -valley. On one occasion I em- 

 ployed a heliotrope at 6^ miles, and used an aperture of one sixth of an 

 inch, and found it rather brighter than usual, so that probably 62 or 7 

 mile's would be the normal distance for that size. But there is no 

 need to employ a conjectural quantity ; and if the rate of absorption 

 corresponding to the above be computed, so close an agreement will 

 be found, as may entitle the numbers to be looked on as something 

 better than mere estimates. The mean of the whole shows a loss of 

 .0610 in passing through one mile of atmosphere ; with the barometer 

 reduced to 30.0 inches, the quantity will be .0671. Hence the loss 

 of light in passing from the zenith through a homogeneous atmosphere 

 of 5.2 miles will be .3J33. I was much astonished at first discovering 

 that the air had so great absorbent powers, and many ideas are sug- 

 gested by the fact. We see at once how easily many of the planets 

 may be rendered habitable to beings like ourselves. Mars may enjoy 

 a temperature little inferior to our own by having a less absorbent en- 

 velope ; and Venus may be kept as cool as we are, by having one more 

 so." Proceedings of the Edinburgh Royal Society, Vol. II. No. 36. 



PROPORTION OF LIGHT PtEFLECTED AND TRANSMITTED. 



ARAGO, in one of the series of papers on photometry which he has 

 lately presented to the French Academy, states that, in the course of 

 some investigations, it became necessary to know the quantity of light 

 transmitted and reflected by a plane plate of glass at various angles, 

 and as this was not to be ascertained from any book, he determined it 

 experimentally. He found that at an angle of 4 32' a plate of crown- 

 glass reflects four times as much light as it transmits ; at 7 1' the 

 reflected light is double the transmitted ; at 1 1 8' the two are equal ; 

 at 17 17' the reflected light is half of the transmitted, and at 26 38' 

 it is a quarter of the transmitted light. Arago then goes on to de- 

 monstrate the law known as the "square of the cosine," and shows 



