ASTROPHYSICS IN THE UNITED STATES BOSLER. 363 



Several spectroscopes of one or several prisms may be attached to 

 the telescope; we will mention especially the Rumford spectro- 

 heliograph. Hale had especially in view the study of the sun when 

 he founded the Yerkes Observatory. So he had the great refractor 

 furnished with adjuncts analogous to those which he had used in 

 his private observatory at Kenwood for his earlier solar researches. 

 Yet we will find that there is now a tendency everywhere to adopt 

 widely different schemes for this class of researches. 



Under the incentive of Prof. Frost, now the director at the Yerkes. 

 progress has, of course, continued. With the 40-inch Burnham made 

 . his great catalogue of double stars published in 1906. This includes 

 many new doubles, which European astronomers may not hope to 

 see. In photography Ritchey, the clever constructor and observer 

 now at Mount Wilson, first applied at the Yerkes the idea of moving 

 the plate holder with its plate (pi. 4) in following a star instead of 

 moving the whole telescope. And yet further, by the use of isochro- 

 matic plates combined with suitable color screens for eliminating the 

 blue and violet rays for which the objective was not corrected pho- 

 tographically, he succeeded in obtaining remarkable plates of star 

 clusters. 4 



Studies in photometry have been carried on at the same time, and, 

 just as at Meudon, daily photographs are taken of the protuber- 

 ances of the sun, and also of the strata of calcium and hydrogen over 

 the whole solar atmosphere. 



The Yerkes Observatory has other instruments: a telescope of 24 

 inches constructed there almost entirely by Ritchey ; finally, through 

 the generosity of Miss Bruce, there is a photographic telescope of 

 25 centimeters linear aperture and 1 to 5 angular aperture of large 

 field, which has enabled Barnard to obtain, besides numerous pho- 

 tographs of the comets which are magnificent, plates of large areas of 

 the milky way and to discover through these latter those large, dark, 

 star-free places, commonly called coal sacks, which so perplex the 

 scientist. 



FLAGSTAFF OBSERVATORY MARS AND THE PLANETARY SURFACES. 



And now we will pass to a class of work which recently has aroused 

 lively curiosity even outside of scientific circles; this is the study of 

 the surfaces of the planets, undertaken especially since 1894 and in 

 particular very recently at the observatory at Flagstaff by Percival 

 Lowell. Mr. Lowell, a rich amateur astronomer, early conceived 

 a passion for studying the question of the " canals " of Mars and 

 has become an ardent and intelligent advocate of the habitability of 

 this enigmatic planet. The Martians seem almost friends of his, he 

 has become so ardent in describing their exploits. In order to better 



