SOME RECENT ASTRONOMICAL EVENTS. 159 



pended by an extronieh' thin quartz fiber. All is iiu'losed in a metal 

 case, with a g-lass window opposite the little mirror, so as to observe 

 the deflections of the vanes by the telescope and scale method, and a 

 second window of fluorite or other material transmissible to the lon^ 

 wave-length radiations is inserted opposite the vanes to admit the rays 

 to bo measured. The case is air-tight and may be exhausted to any 

 degree. The sole force which keeps the vanes at the zero of position 

 when uninfluenced by radiation is the tortional elasticit}^ of the quartz 

 fiber, and this resists the rotation of the vanes and returns them to 

 their original position when turned temporarily from it by the influ- 

 ence of radiation. 



An extraordinary degree of sensitiveness of this instrument was 

 indicated by experiments which were made on the heat of a candle sit- 

 uated 2,000 feet from the concave mirror which focused its rays upon 

 the radiometer. The feeble radiations of the candle at this great dis- 

 tance sufficed to turn the radiometer through nearly a hundred scale 

 divisions, and even the face of an oliserver, when placed in the position 

 before occupied by the candle, produced a deflection of 25 scale divi- 

 sions. As a tenth of a single scale division could readily be observed, 

 it will be seen, to speak figuratively, that with the radiometer one 

 might note the approach of a friend while yet some miles distant, 

 merely by the glow of his countenance. 



Cbrrecting the observation upon the candle for the al)sorptiou of the 

 earth's atmosphere in the layer between it and the radiometer, it was 

 found that in the absence of the atmosphere, a single candle at upward 

 of 16 miles could have been detected, so that the instrumental equip- 

 ment was far more sensitive than that used by Professor Boys in the 

 negatively resulting stellar observations alread}' alluded to. 



Experiments were performed upon the radiations of the stars Vega 

 and Arcturus, and on the planets Jupiter and Saturn. The heat 

 of each of these objects was distinctly recognized, and caused, in 

 the mean, deflections of 0.51, 1.11:, 2.38, and 0.37 scale divisions, 

 respectively, when approximate!}' reduced to zenith. Thus the rela- 

 tive thermal efi'ects of Vega, Arcturus, rlupiter, and Saturn are as 

 1:2.2:4.7:0.71. This, it will be seen, is quite appreciably diflerent 

 from thcnr relative brightness to the eye, a circumstance which may, 

 with additional experiments, lead to interesting conclusions regarding 

 the nature of the radiation received from these sev^eral objects. 



4. THE OBSERVATIONS OF THE PLANET EROS. 



The minor planet Eros, it will be recalled, was discovered ])y Witt, 

 of the Urania Observatory at Berlin, August 13, 1898. When after 

 several observations its approximate orbit was computed, this was 

 found to be so highly eccentric as to differentiate this new planet from 

 the many other asteroids with which it had been provisionally classed. 



