40 



equipment, and he introduced the custom of observing a 

 star directly with one circle, but by reflection from the 

 surface of mercury in the other. 



The present Transit Circle, the fourth of this memorable 

 series, is, in effect, a combination of the mural circle with 

 the old form of transit, but is more powerful and capable of 

 a greater delicacy of observation than its predecessors, 

 while its errors of adjustment and form can be far better 

 ascertained and allowed for. Its object-glass is 8-1 inches 

 in diameter, figured by WILLIAM SIMMS (1793-1860), partner 

 of Edward Troughton (TROUGHTON AND SIMMS), and the 

 instrument itself was constructed by RANSOMES AND MAY, 

 the specification requiring that the pivots should be true to 

 jo-Joo-th of an inch. The divided circle is 6 feet in diameter, 

 and is read by seven microscopes, fixed in the west pier. 

 By the use of the chronograph the mean time of the transit 

 of an equatorial star can be determined to about a hundredth 

 of a second of time, and its zenith distance, to the tenth 

 part of a second of arc. 



These are refinements quite outside anything that is 

 required for the practical purposes of navigation, and far 

 beyond the possibilities of observation with the sextant at 

 sea. The work of the Observatory, therefore, in this direction 

 would have entirely passed beyond its purely maritime 

 purposes even if the great improvement in the performance 

 of chronometers had not superseded the use of " lunars " 

 in ordinary navigation. 



The progress in the numbers of the stars of which the 

 places are determined is even more remarkable than the 

 advance in precision. Maskelyne's observing list included 

 only some 36 stars, but during the course of the igth century 

 programmes of stellar observation were set on foot on a scale 

 some 20,000 times as extensive. Argelander at the Bonn 

 Observatory observed the places of 324,000 stars north of 

 South Dec. 2, and Schonfeld, Gould and Thome extended 

 it to the South Pole. These catalogues embraced stars 

 down to those only half as bright as the Qth magnitude, 

 but some astronomers found it necessary to observe stars 

 still fainter and surveyed portions of the sky down to the 

 I4th magnitude. But the Milky Way is so rich in the fainter 

 orders that here the number of stars claiming record baffled 

 all the accustomed means of observation. The application 

 of gelatine plates to photography, however, supplied the 

 solution of the difficulty, and in 1887 a great conference of 

 astronomers met at Paris to determine upon an international 



