134 PROGKESS IN ASTRONOMY. 



clock near at hand and estimating the fractions. Some observers con- 

 stantly note the time either a little in advance or a little later than the 

 actual time, and this small distance between the oljserver and the true 

 times is more or less constant for each observer. This difference has 

 to be taken into account for every observation. Even the use of the 

 chronograph in transit work, by which the observation is electrically 

 recorded, does not entirely" eliminate the error. The photographic 

 method of transit work has been experimented on, but so far as I know 

 it has not yet been used at more than one or two observatories. It 

 will doubtless eventually rid us of "'personal equation" entirel}", for 

 the star image may be photographed and the time recorded by the same 

 current of electricity. 



At the end of the century Ve ma}' almost say that, except in relation 

 to the work of the meridional observatories, photographic methods of 

 recording observations are becoming exclusively used. One of the 

 cases in which its utilit}' is most in evidence is in the matter of eclipse 

 observations. Spectra of the sun's surroundings containing a thousand 

 lines are taken in a second of time, thus replacing five or six doubtful 

 eye observations by wealth of results, which have enabled the recent 

 vast progress to be secured. 



CATALOGUES. 



Catalogues of the stars were among the first scientific records started 

 by man, and so long as onl}' the naked eye was used the work was not 

 difiicult, as onl}'^ approximate positions were attempted, even by Hip- 

 parchus; but long before the eighteenth century dawned the problem 

 was entirely changed by the invention of the telescope and by the pro- 

 vision of accurately divided circles; not only could better positions be 

 recorded, but the number of stars to be catalogued was enormously 

 increased, and furthermore other objects — nebulaj — presented them- 

 selves in considerable numbers. 



In 1801 the star catalogues chiefl}" relied on were those of Lacaille, 

 containing about .3,000 stars scattered over the whole heavens. 



Maskelyne, who was then astronomer ro3'al, had published in 1790 

 a catalogue of thirtj^-six fundamental stars, chiefly for the purposes 

 of navigation. The first great catalogue of the century was the Fun- 

 damenta AstronomiBe of Bessel, produced in 1818. This contained 

 3,222 stars. The Bonn "Durchmiisterung/' with its catalogue of 

 331,198 stars in the northern hemisphere and the corresponding atlas 

 published in 1857-1863, was the next memorable achievement in this 

 direction. For it we have to thank Bessel and Argelander and a per- 

 fect system of work. 



Another monumental catalogue dealing with the stars in the south- 

 ern heavens has been that of the southern stars observed ))v Gould 



