70 REPORT OF THE SECRETARY. 



(f ) Observalions on miscellaneous subjects. — Distribution of energy from terrestrial sources. 



A lumiber of energy curves, some of which are here given, were taken, in which.the 

 Kitson lamp, provided with mantles of various kinds, was the source. Among the 

 mantles tried were the ordinary Welsbach (which consists of impure thorium oxide) 

 and others composed of jiure thorium oxide, iron oxide, uranium oxide, etc. ' The 

 distribution of energy between different wave lengths with these sources, so different 

 in illuminating power, is much less diversified than would be supposed, and goes 

 strongly to show the wastefulness even of the Welsbach light as a source of illumina- 

 tion. For the invisible infra-red in all cases includes by far the major portion of the 

 energy, and not the visible spectrum, as is the case with the sun and still more with 

 phosphorescent substances. However, by the employment of a second spectroscope, 

 or "sifting train," to exclude the stray infra-red radiations, we were able to determine 

 the distribution of the relatively small amount of energy in the visible spectra of 

 the various sources, and to show how far the ordinary Welsbach mantle outstripped 

 them all for light, especially in the red, orange, and yellow. 



In these experiments a very considerable number (at least fifty) of absorption 

 bands were discovered at wave lengths beyond -i/i, which were most probably due 

 to the gases given off by the lamp in burning, and perhaps solely to carbon dioxide. 



Absorption in. the solar spectrum. — All the holographic records, extending back to 

 1893, were carefully examined with regard to the changes in absorption noted in last 

 year's report, and such changes were found to be more extensive and frequent than 

 had been supposed. The great decreases in absorption at W and Di were found to 

 occur every spring, and to a lesser extent every fall; but were occasionally found in 

 the winter also, but never in the summer. Such changes were found sometimes to 

 go through their whole cycle in a week, and the absorption here is fomid to largely 

 increase with declining sun. 



The effect of water to absorb in this region was studied. Narrow cells of glass, 

 whose absorption was known, were tilled with water and placed in the path of the 

 beam in taking holographs of the solar spectrum. It was thus shown that " liquid " 

 water absorbs most strongly at the particular regions where these annual variations 

 are noticed. A fraction of a milhmeter thickness of water was found enough to 

 produce a great effect beyond the wave length 1.2 jj., and 2 millimeters thickness pro- 

 duced almost complete absorption of the solar rays beyond 1.2/^. 



Constants of prisms. — In the course of the reduction of measures on holographs the 

 exact index of refraction at the A line for the salt and fluorite prisms was required. 

 Several measurements of these quantities made at different times failing of satisfac- 

 tory agreement, several interesting things came out in finding the sources of error. 



Rock salt prisms have not constant angles. It was shown that with a rising 

 temperature the faces of the prism, if at first flat, became convex, and all three 

 angles of the prism increase, unless they be determined at the centers of the faces. 

 After the discovery of the very appreciable value of this change with our great 

 prisms, it was at once fomid that their faces, when polished flat, are considerably 

 concave after coming to their constant temperature. This is because of the surface 

 heating caused b}- the friction of the polisher. It is now the jsractice to leave the 

 faces polished very slightly convex, to allow for this temperature change. 



Effect of this on tlie accuracy of holographs and on the determination of prism 

 angles. After a very considerable amount of analytical investigation it was shown 

 that such concavity of prism faces as was present when the holographs were taken 

 need introduce no error of appreciable magnitude in the relative deviations of the 

 holographs, provided the angle of the prism was determined to within 10 seconds of 



' The Observatory is indebted to Mr. Waldron Shapleigh, chemist of the Welsbach 

 Company, for valuable advice and material used m this research. 



