ON THE MAGNITUDE OF THE SOLAR SYSTEM. 109 



The prevailiug opinion certainly is tliat great advances have recently 

 been made in astronomy, and so they have in the fields of spectral 

 analj^sis and in the measnreraent of minute quantities of radiant heat; 

 but the solution of the vast majority of astronomical problems depends 

 upou the exact measurement of angles, and in that little or no progress 

 has been made. Bradley, Avith his zenith sector a hundred and fifty 

 years ago, and Bessel and Struve, with their circles and transit instru- 

 ments seventy years ago, made observations not sensibly inferior to 

 those of the present day, and indeed it would have been surprising if 

 they had not done so. The essentials for accurately determining star 

 places are a skilled observer, a clock, and a transit circle, the latter 

 consisting of a telescope, a divided circle, and four micrometer micro- 

 scopes. Surely no one will claim that we have to-day any more skillful 

 observers than were Bessel, Bradley, and Struve, and the only way in 

 which we have improved upon the telescopes made by Dolloud one 

 hundred and thirty years ago is by increasing their aperture and rela- 

 tively diminishing their focal distance. The most famous dividing 

 engine now in existence was made by the elder Repsold seventy-five 

 years ago; but as the errors of divided circles and their micrometer 

 microscopes are always carefully determined, the accuracy of the meas- 

 ured angles is quite independent of any small improvement in the 

 accuracy of the divisions or of the micrometer screws. Only in the 

 matter of clocks has there been some advance, and even that is not 

 very great. On the whole, the star places of to-day are a little better 

 than those of seventy-five years ago, but even yet there is great room 

 for improvement. One of the commonest api:)lications of these star 

 places is to the determination of latitude, but it is very doubtful if 

 there is any point on the face of the earth whose latitude is known 

 certainly within one-tentb of a second. 



Looking at the question from another point of view, it is notorious 

 that the contact observations of the transits of Venus in 17(51 and 1769 

 were so discordant that from the same observations Encke and B. J. 

 Stone got respectively for the solar parallax 8.59" and 8.91". In 1870 

 no one thought it possible that there could be any difficulty with the 

 contact observations of the then approaching transits of 1874: and 1882, 

 but we have found from sad experience that our vaunted modern instru- 

 ments gave very little better results for the last pair of transits than 

 our predecessors obtained with much cruder appliances in 1761 and 1769. 



The theory of probability and uniform experience alike show that the 

 limit of accuracy attainable with any instrument is soon rea(;hed; and 

 yet we all know the fascination which continually lures us on in our 

 efforts to get better results out of the familiar telescopes and circles 

 which have constituted the standard equipment of observatories for 

 nearly a century. Possibly these instruments may be capable of indi- 

 cating somewhat smaller quantities than we have hitherto succeeded 



