191 1.] ABBOT— SOLAR CONSTANT OF RADIATION. 239 



zenith distance.' Solar-constant determinations require no higher 

 values of zenith distance than these to be considered. 



Radau and Langley proved the necessity of confining the atmo- 

 spheric use of Bouguer's formula to approximately monochromatic 

 rays. In general, for a reason which Lord Rayleigh has shown, the 

 transmission of the atmosphere increases gradually with increasing 

 wave-lengths. Thus in the violet the transmission for a vertical ray 

 to sea level may be 50 per cent., and for a deep red ray 80 per cent. 

 But besides this gradual change there are also spectral regions of 

 almost complete absorption by atmospheric oxygen, and by water- 

 vapor, so that in these regions the transmission approaches zero. 

 If we should disregard these differences, and determine the con- 

 stants of the exponential formula above, by pyrheliometric measure- 

 ments alone at dift'erent solar zenith distances, our result E^ for 

 the intensity outside the atmosphere must necessarily be too small.* 



Langley was the first to act upon this, and 'to devise apparatus 

 and methods for measuring the energy and the atmospheric trans- 

 mission at all parts of the spectrum. For this purpose he invented 

 the bolometer about 1880, and automatic registration of its indica- 

 tions about 1890. As we now use it the bolometer comprises two 

 similar tapes of platinum, each about i cm. long, o.oi cm. wide and 

 o.ooi cm. thick. These are coated with lamp-black by smoking over 

 a camphor flame. They lie parallel to the spectrum lines, and 

 about 0.8 cm. apart. One tape may be shined upon by the rays, 

 the other can not. Hence the heat absorbed from a narrow region 

 of spectrum, usually about twice the extent comprised between the 

 D lines, raises the temperature of the exposed tape with reference to 

 the other. The two tapes and two resistance coils are combined to 

 form a Wheatstone's bridge, and the rise of temperature produced 

 as above stated deflects a sensitive galvanometer. The galvanometer 

 needle reflects a tiny spot of light on a photographic plate, which 

 moves vertically as driven by clock work. The same clock work 

 moves the spectrum slowly over the bolometer tape. In this way 

 may be produced in from eight to twelve minutes, according to the 

 spectroscopic outfit employed, a holograph, or spectrum energy 



' Loc. cit., p. 59. 

 * Loc. cit.. p. 16. 



