58 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



11. Finally, in following out this line of thought, the spectra- 

 bolometer represents the present limit of thoroughness. By measur- 

 ing the heat energy of every part of the spectrum, all of the results 

 may be expressed in the same absolute terms, whereas the result- 

 obtained by photochemical reactions must be in different terms for 

 each of the several reactions which are required to cover the spec- 

 trum. Furthermore, bolometric observations permit the comparison 

 of deficiencies in any region with deficiencies in the whole energy of 

 the light as determined by the same means. As has been pointed 

 out. the heat energy in the region of greatest chemical activity is 

 small; but it is not too small for precise measurement, and when 

 once measured it may be transposed to terms of definite chemical 

 reactions, the transposition factor varying, of course, with each 

 wave length and with each reaction considered. 



The Langley bolometer has been briefly mentioned, because it was 

 designed for measurements of the whole energy of sunlight. The 

 following description from Baly (52) indicates the manner in which 

 the same principle was adapted to the most minute quantities. 



In his final work upon the solar spectrum. Langley made use of a new apparatus; 7 

 the light from a 20-inch siderostat passed through the slit of a horizontal collimator, 

 which possessed a lens of rock salt 17 centimeters clear aperture and L0 meters focal 

 length. This lens focused the ray upon a prism or grating; the prism was of rock 

 salt, and was 18.5 centimeters high and 12 centimeters deep in the face, and Lad a 

 refracting angle of 60°. The angular width of the bolometer thread was decreased 

 to 2 inches of arc by using a telescope lens of 5 meters focus; the sensitiveness was 

 thereby increased, and by improvements in the galvanometer the apparatus was 

 made capable of detecting a temperature change of 0.000001° C. The whole spec- 

 trometer was of the fixed-arm type, and the spectrum was made to pass over the 

 bolometer strip by rotating the prism. An automatic self-registering method was 

 adopted of recording the galvanometer readings. The spot of light reflected from 

 the galvanometer mirror was focused upon a broad strip of photographically sensitive 

 paper. This paper strip was caused to move slowly in a vertical direct ion. and in 

 this way a faithful record of the excursions of the light spot was obtained. At the 

 same time the prism was slowly rotated, and therefore this record clearly sin. wed all 

 the temperature changes of the bolometer as the spectrum passed over it. Further, 

 the motions of the sensitized paper and the prism were exactly coordinated, so that 

 the angular position of the prism corresponding to any portion of tin- galvanometer 

 record could at once be obtained. In this way, since the dispersion of the prism 

 was already known, the wave length of any spectrum line shown upon the record 

 could be found, and also, from the length of the throw of the light spot, its intensity 

 estimated. The delicacy of this apparatus was sufficient to show the I> linos widely 

 separated, with the nickel line in between. * * * 



o By means of this apparatus, Langley mapped the solar spectrum as far as 55 000 

 Angstrom units, and observed 700 lines between A and this limit. 



12. Evaporimeters may be used for a very rough measure of the 

 heatmg value of sunlight. At first thought it would seem thai the 

 rate of evaporation would be an almost ideal measure, since the 



7 Brit. Ass. Rep., 1894, p. 465; and Nature, 51, 12 (1894). 



