218 W. Ilarhiess — Magnitude of the Solar System. 



The prevailing opinion certainly is that great advances have 

 recently been made in astronomy, and so they have in the 

 fields of spectral analysis and in the measurement of minute 

 quantities of radiant heat ; but the solution of the vast major- 

 ity of astronomical problems depends upon the exact measure- 

 ment of angles, and in that little or no progress has been 

 made. Bradley with his zenith sector a hundred and fifty 

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

 instruments 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 microscopes. 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 Dol- 

 lond one hundred and thirty years ago, is by increasing their 

 aperture and relatively 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 measured 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 com- 

 monest applications 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-tenth 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 1761 and 1769 were so discordant that from the same 

 observations Encke and E. J. Stone got respectively for the 

 solar parallax 8'59 seconds and 8 - 91 seconds. In 1870 no one 

 thought it possible that there could be any such difficulty with 

 the contact observations of the then approaching transits of 1874 

 and 1882, but now we have found from sad experience that 

 our vaunted modern instruments 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 reached ; and ■ yet we all know the fascination which 



