80ME RECENT ASTRONOMICAL EVENTS. 157 



poiiitino- toward the celestial ol)ject, l)ut pointiiio- rather at a g-reat 

 mirror which itself reflected the light to the lens. This combination 

 of the lens and the mirroi" is coming increasingly into favor. It was 

 used with advantage, as the readers of last year's report remem])er, 

 by the Smithsonian eclipse expedition of lUOO, and with no less success 

 by observers of that eclipse from other places, notably by Professor 

 Barnard of the Yerkes expedition. 



The advantage of this ariangement consists chiefly in that the tele- 

 scope is immovable and therefore not so much su])ject to the shaking 

 of the ground, bad following of the clock, or to flexure of the tube 

 of the lens or of the lens itself, all of which are liable to seriouslv 

 affect the steadiness and perfection of the imag'e of a great equatorial. 

 Of course these sources of error all come in to disturb the reflecting' 

 mirror which is placed in front of the telescope; l)ut j^et. owing* to 

 the compactness and relatively' small weight of the apparatus which is 

 tluM'e driven, these sources of error may ))e nuich diminished. Besides 

 these advantages we have the not inconsiderable furthei gain that the 

 visual or photographic observer can carry on his operations with per- 

 fect comfort and convenience, owing to avoiding the necessity of fol- 

 lowing the moving eye end of an equatorial. In a recent visit to the 

 Yerkes Observatory I had the pleasure of seeing the beginnings of 

 very large telescopes of this pattern which Professor Hale designs to 

 emi)loy for the most delicate and far-reaching photographic and radio- 

 metric investigations. 



3. THE MEASUREMENT OF THE HEAT RECEIVED FROM THE STARS. 



Attempts were made as early as ISOl) and ISTO b}^ English astrono- 

 mers to obtain evidence of the heat received at the earth from the 

 l»rightest stars. These ex})erLments were carried out with the aid of 

 the thermopile, then the most sensitive form of heat-measuring appa- 

 ratus known. 



Since 1880 there have been devised, howev^er, as man}' as four instru- 

 ments far more sensitive than the old-fashioned thermo[)il(\ These 

 are the bolometer, the radiomicrometer. the improved thermopile of 

 Rubens, and the radiometer, which last has reached its greatest sensi- 

 tiveness in the hands of Prof. E. F. Nichols. 



In 1888 Prof. C. B. Boys, with his then newl}' invented I'adiomi- 

 crometer, repeated the earlier observations on the heat of the brighter 

 stars, and while the earlier observers had convinced themselves of dis- 

 cernible heating effects, he, with his far more sensitive arrangements, 

 came to negative results. As showing the great sensitiveness of his 

 apparatus and the therefore extreme minuteness of the amount of heat 

 received from the stars, it need only be said that in the absence of 

 atmospheric absorption a candle placed at almost 2 miles distance 

 would have been perceived by him. 



