376 ASTRONOMY 



light of the stars, when only the naked eye or a telescope is available, 

 and much valuable work might be done by applying it to the fainter 

 stars, and especially to clusters. 



Meanwhile photometric measures of the stars according to various 

 methods had been undertaken. In 1856, Pogson showed that the 

 scale of magnitudes of Ptolemy, which is still in use, could be nearly 

 represented by assuming the unit to be the constant ratio, 2.512, 

 whose logarithm is 0.4. This has been generally adopted as the basis 

 of the standard photometric scale. The photometer devised by Zoll- 

 ner has been more widely used than any other. In this instrument, 

 an artificial star is reduced any desired amount, by polarized light, 

 until it appears to equal the real star, both being seen side by side in 

 the telescope. Work with this instrument has attained its greatest 

 perfection at the Potsdam Observatory, where measures of the light 

 of the northern stars, whose magnitude is 7.5 and brighter, have been 

 in progress since 1886. The resulting magnitudes have been published 

 for 12,046 stars, included in declination between 2 and +60. 

 The accidental errors are extremely small, but as the results of differ- 

 ent catalogues differ systematically from one another, we cannot be 

 sure which is right and what is the real accuracy attained in each 

 case. In 1885, the Uranometria Oxoniensis was published. It gives 

 the magnitudes of 2784 northern stars, north of declination 10. 

 This work is a remarkable one, especially as its author, Professor 

 Pritchard, began his astronomical career at the age of sixty- three. 

 The method he employed was that of reducing the light of the stars by 

 means of a wedge of shade glass until they became invisible, and then 

 determining the brightness from the position of the wedge. A careful 

 and laborious investigation, extending over many years, has been car- 

 ried on by Mr. H. M. Parkhurst, using a modification of this method. 



For several years before the Oxford and Potsdam measures de- 

 scribed above were undertaken, photometric observations were in 

 progress at Harvard. In 1877, a large number of comparisons of 

 adjacent stars were made with a polarizing photometer. Two images 

 of each star were formed with a double-image prism, and the relative 

 brightness was varied by turning a Nicol prism until the ordinary 

 image of one star appeared equal to the extraordinary image of the 

 other. Several important sources of error were detected, which, once 

 known, were easily eliminated. A bright star will greatly affect the 

 apparent brightness of an adjacent faint one, the error often exceed- 

 ing a magnitude. Systematic errors amounting to several tenths of 

 a magnitude depend upon the relative positions of the images com- 

 pared. They are perhaps due to the varying sensitiveness of the 

 different parts of the retina. This photometer has many important 

 advantages. However bad the images may be, they are always exactly 

 alike, and may, therefore, be compared with accuracy. Both stars are 



