136 Intelligence and Miscellaneous Articles. 



PRELIMINARY NOTE ON THE ENERGY SPECTRUM OF A BLACK 

 BODY. BY C. E. MENDENHALL AND F. A. SAUNDERS. 



In the Astrophysical Journal for August, 1895, Dr. H. F. Beid 

 suggested the experimental study of this subject, based upon the 

 theoretical deductions of Stewart and Kirchoff, that the radiation 

 existing in an enclosure with walls at a uniform temperature 

 would be independent of the character of the walls and would be 

 that of an absolutely black body at that temperature, provided 

 only that the walls had at least an infinitesimal absorptive power 

 for all wave-lengths. This work we have been carrying on, and 

 though it is not completed, enough has been obtained to warrant 

 a brief account. 



In order to study the radiation iu the interior of a closed body 

 it is of course necessary to cut a small slit iu its walls, and thus 

 the theoretical conditions cannot be absolutely fulfilled ; then, 

 again, it is practically impossible to obtain an absolutely uniform 

 heating of a body of considerable size. If it is found, however, 

 that varying the size of the slit does not change the form of the 

 curve, then we can be reasonably sure that the first of these diffi- 

 culties is au unimportant one. This point is now under investi- 

 gation. As regards the non-uniformity of temperature it is 

 reasonable to suppose that any curve will be a meau of the curves 

 corresponding to several adjacent temperatures, and that it will 

 not, in form, depart far from any one of them, if the departure 

 from uniformity is not large. 



The bodies we have used are cylinders of iron and of copper, 

 6 inches high and 4| inches inside diameter, and the ratio of the 

 area of slit to the total inside area is ^i-^-. One arm of the 

 spectrometer reaches to within 3 inches of the furnace and carries 

 the real optical slit which is 1*7 millim. wide. The radiation 

 through this slit falls upon a rock-salt prism with faces 2x3 inches 

 and th n upon a h-ns of the same material, 3 inches in diameter, 

 which focusses it upon the bolometer strips, which are of platinum 

 foil, coated with platinum black, and these subtend an angle of 

 five minutes at the axis of the spectrometer. The galvanometer 

 is a four-coil oue, has a resistance of three ohms, connected in 

 parallel, and gives a sensibility of about 5 x 10" 10 for 10 sec. complete 

 period and a metre distance. 



As far as our preliminary results show there is between 1050° 

 and 500° C, a noticeable shift of the maximum of the energy 

 spectrum towards the blue with rise of temperature. An absorp- 

 tion band, not due to Mater or to carbon dioxide, falls very near 

 the maxima of our curves and often prevents these from being 

 directly observed ; this is probably due to gases filling the black 

 body, entering from the gas-burner by the holes through which 

 pass the platinum-platinum-iridium thermocouples used iu mea- 

 suring the temperature of the black body. — Johns Hopkins Uni- 

 versity Circulars, June 1897. 



