386 POPULAR SCIENCE MONTHLY. 



This, however, was far from being the most difficult part of the enter- 

 prise. The most arduous task of measuring the positions of a half- 

 million of stars on the negatives, including the determining of the 

 magnitude of each, was undertaken by Professor J. C. Kapetyn, of the 

 University of Groningen, Holland, and brought to a successful com- 

 pletion in the year 1899. 



What the work gives is, in the first place, the magnitude and ap- 

 proximate position of every star photographed. The determining of 

 the magnitude of a star is an important and delicate question. There 

 is no difficulty in determining, from the diameter of the image of the 

 star as seen in the microscope, what its photographic magnitude was 

 at the time of the exposure, as compared with other stars on the same 

 plate. But can we rely upon similar photographic magnitudes on a 

 plate corresponding to similar brightnesses of the stars? In the opinion 

 of Gill and Kapetyn we cannot. The transparency of the air varies 

 from night to night, and on a very clear night the same star will give 

 a stronger image than it will when the air is thick. Besides, slightly 

 different instruments were used in the course of the work. For these 

 reasons a scale of magnitude was determined on each plate by com- 

 paring the photographic intensity of the images of a number of stars 

 with the magnitudes as observed with the eye by various observers. 

 Thus on each plate the magnitude was reduced to a visual scale. 



It does not follow from this that the magnitudes are visual, and not 

 photographic. It is still true that a blue star will give a much stronger 

 photographic image than a red star of equal visual brightness. In a 

 general way, it may be said that the catalogue includes all the stars to 

 very nearly the tenth magnitude, and on most of the plates stars of 

 10.5 were included. In fact, now and then is found a star of the eleventh 

 magnitude. 



A feature of the work which adds greatly to its value is a careful 

 and exhaustive comparison of its results with previous catalogues of the 

 stars. When a star is found in any other catalogue the latter is indi- 

 cated. Most interesting is a complete list of catalogued stars which 

 ought to be on the photographic negatives, but were not found there. 

 Every such case was inexhaustibly investigated. Sometimes the star 

 was variable, sometimes it was so red in color that it failed to impress 

 itself on the plate, sometimes there were errors in the catalogue. 



The great enterprise of making a photographic map of the heavens 

 now being carried on as an international enterprise, having its head- 

 quarters at Paris, is yet wider in its scope than the works we have just 

 described. One point of difference is that it is intended to include all 

 the stars, however faint, that admit of being photographed with the 

 instruments in use. The latter are constructed on a uniform plan, the 

 aperture of each being 34 centimetres, or 13.4 inches, and the focal 



