48 POPULAR SCIENCE MONTHLY. 



Meanwhile photometric measures of the stars, according to various 

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

 of magnitudes of Ptolem} r , which is still in use, could be nearly repre- 

 sented 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 Zollner has 

 been more widely used than any other. In this instrument an arti- 

 ficial 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 per- 

 fection 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 can not 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 mag- 

 nitudes of 2,784 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 em- 

 ployed was 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 carried 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 prog- 

 ress 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 bright- 

 ness was varied by turning a Nicol prism until the ordinary image of 

 one star appeared equal to the extraordinary image of the other. Sev- 

 eral important sources of error were detected, which once known were 

 easily eliminated. A bright star will greatly affect the apparent bright- 

 ness of an adjacent faint one, the error often exceeding a magnitude. 

 Systematic errors amounting to several tenths of a magnitude depend 

 upon the relative positions of the images compared. 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 affected equally by passing 

 clouds, so that this photometer may be used whenever the stars are 

 visible and at times when other photometric work is impossible. The 

 diminution in light also follows a simple geometrical law, and is readily 

 computed with great accuracy. There is no unknown constant to be 



