CHAPTERS ON THE STARS. 237 



most that can be done is to bring abont the best attainable agreement 

 between the two systems in the general average of all the stars. 



Fortunately the differences between the colors of the stars are by 

 no means so great as those between the colors of natural objects around 

 us. All the stars radiate light of all colors; and although the difference 

 is quite appreciable either by the eye or by the photograph, it is not 

 so great as it would have been were the variations in color as wide as in 

 the case of terrestrial objects. 



Two comprehensive surveys of the heavens, intended to determine 

 as accurately as possible the magnitudes of all the brighter stars, have 

 recently been undertaken. One of these is the Harvard photometry, 

 commenced by Professor Pickering at the Harvard Observatory, and 

 now extended to the Southern Hemisphere by the aid of a branch 

 establishment at Arequipa, Peru. 



The instrument designed by Professor Pickering for his purpose is 

 termed a meridian photometer, and is so arranged that the observer 

 can see in the field of his telescope a reflected image of the Pole Star, 

 and, at the same time, the image of some other star while it is passing 

 the meridian. By a polarizing apparatus the image of the star to be 

 measured is made to appear of equal brightness with that of the Pole 

 Star, and the position of a Nicol prism, which brings out this equality, 

 shows the ratio between the magnitudes of the two stars. 



The other survey, with the same object, is now being made at the 

 Potsdam Astrophysical Observatory, near Berlin. In the photometer 

 used by the German astronomers the image of one star is compared with 

 an artificial star formed by the flame of a candle. The work is per- 

 formed in a more elaborate way than at the Harvard Observatory, and 

 in consequence, only that part of the heavens, extending from the 

 equator to 40° north declination, has been completed and published. 

 A comparison of the. results thus obtained with those of Professor 

 Pickering, shows a curious difference depending on the color of the 

 star. In the case of the reddest stars, the estimates are found to be 

 in fairly close agreement, Pickering's being a little the fainter. But 

 in the case of the white or bluish stars, the estimates of the German 

 astronomers are more than one fourth of a magnitude greater than those 

 of Pickering. This corresponds to an increase of nearly one fifth in the 

 brightness. Whether this difference is to be regarded as purely psycho- 

 logical or due to the instruments used, is an interesting question which 

 has not yet been settled. It is difficult to conceive how different in- 

 struments should give results so different. On the other hand, the com- 

 parisons made by the Germans make it difficult to accept the view that 

 the difference is due purely to the personality of the observers. There 

 are two German observers, Drs. Midler and Kempf, whose results agree 

 with each other exactly. On the other hand, Pritchard, at Oxford, 



