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THE POPULAR SCIENCE MONTHLY. 



comparisons up to the present century hav- 

 ing been made either by the naked eye, or 

 with the assistance of an opera-glass only. 

 About the middle of this century photom- 

 eters specially adapted for comparing the 

 light of the stars were first used by German 

 astronomers. Zollner invented a photom- 

 eter, consisting of a telescope in which the 

 light from a kerosene-lamp, admitted through 

 a very small hole and presenting the ap- 

 pearance of a star, is compared with the real 

 star under observation. C. S. Pierce, of the 

 United States Coast Survey, used such a one 

 in the construction of a photometric cata- 

 logue of 494 stars, published in Vol. IX of the 

 " Annals of Harvard College Observatory." 

 His description of his difficulties with this 

 very imperfectly contrived and still more 

 imperfectly constructed instrument would 

 be amusing, if it did not excite regret that 

 so accurate an observer and excellent a 

 mathematician should have been weighted 

 with an instrument so poorly adapted to the 

 work. 



Up to within a few years, only about 

 500 stars had been the subject of photo- 

 metric observation, and for some time it has 

 been regarded as highly desirable that sys- 

 tematic comparisons should be made of the 

 light of all stars visible to the naked eye. 



In Vol. XIV of the "Annals of the Ob- 

 servatory of Harvard College " we have the 

 records of the most extensive and complete 

 photometric observations ever undertaken. 

 The great attention which Professor Pick- 

 ering, the director of the observatory, has 

 given to astro-photometry, and the large 

 experience he has had with photometers, 

 both of his own and others' construction, 

 peculiarly fitted him for such a task. 



Part I of the " Annals " opens with a 

 description of the meridian photometer 

 devised by Professor Pickering. Having 

 ascertained by experiment that any change 

 of position on the part of the observer had 

 an injurious effect upon the observations, 

 he constructed his instrument in the form 

 of a broken transit, in which, the line of 

 sight being always horizontal, stars at all 

 altitudes could be observed without moving 

 the head. Experience with other photom- 

 eters had also satisfied him that no arti- 

 ficial light could be a proper standard of 

 comparison for the light of a star. A real 



star was therefore chosen, and the fact that 

 the pole-star is always visible, and its light, 

 on account of its very slight changes of alti- 

 tude, a constant quantity, directed him in 

 his choice of it as the standard of compari- 

 son for all stars. To make sure that it was 

 otherwise suitable, a large number of ob- 

 servations were made of the pole-star, to 

 ascertain if its light was subject to any 

 periodical variation. 



A reliable instrument and perfect stand- 

 ard having been thus obtained, and many 

 preliminaries settled which it is impossible 

 to touch upon here, observations were begun 

 in October, lS^, and continued till Septem- 

 ber, 1882. During this period seven hun- 

 dred series of observations were made, in- 

 cluding 94,4*76 separate comparisons, the 

 result being that every star not fainter than 

 the sixth magnitude, between the north 

 pole and thirty degrees of south declination, 

 was compared from three to fourteen times 

 with the pole-star. The whole number of 

 stars thus compared is 4,260. 



The space to which this review is neces- 

 sarily restricted renders it impossible to 

 give even a passing notice to the immense 

 amount of work expended upon the subject 

 of astro-photometry as recorded in these 

 "Annals." An idea can be formed from the 

 statement that there are in all ninety-one 

 tables, some of which occupy several pages. 

 The " General Catalogue," constituting 

 Table XXVII, alone occupies 211 pages, 

 each line having twenty-six columns, one of 

 which has fourteen sub-headings. 



Part II of the "Annals," the publication 

 of which has been delayed till the present 

 year, is largely devoted to a discussion of 

 the work of those astronomers, from Ptolemy 

 to the present day, whose estimates of the 

 relative magnitudes of the stars Professor 

 Pickering has chosen for comparison with 

 his own. A very complete list of all known 

 or suspected variable stars is also given, with 

 copious notes as to dates and observers. 

 There is also a chapter on the distribution 

 of the stars. 



Professor Pickering does not close his 

 work with the advancement of any theories 

 of his own. But, for whatever purpose an 

 exact determination of the relative magni- 

 tudes of the stars may be desired, either 

 for the discovery of variable stars, or to as- 



