58 ANNUAL RECOKD OF SCIENCE AND INDUSTRY. 



its micrometer screws, so as to be seen on the cross-threads 

 in the focus of A. If the mirror is truly perpendicular to the 

 axis of the mural circle, and that axis is j^erfectly regular, 

 the telescope, M, might be turned in any position, and the 

 mark would still be on the cross-threads of A. If, however, 

 in a given position of M the mark is not on these threads, 

 the micrometer screws of the mark will serve to brins^ it 

 back to these threads, and in this way it may be determined 

 how far the normal to the surface of the mirror is displaced 

 horizontally and vertically for each position of M. In this 

 way the position of M may be referred to a vertical plane 

 passing through the north and south collimators of the mural 

 circle. The ordinary formula for the transit instrument will 

 serve for this purpose if, first, the angle formed by the optical 

 axis of M with the fixed vertical plane (if the mirror is put 

 into that plane) be substituted for the error of collimation ; 

 second, if for the error of azimuth be substituted the angle 

 between the vertical plane through the normal to the mirror 

 and the vertical plane perpendicular to the plane of reference; 

 third, if for the error of inclination be substituted the angle 

 between the normal to the mirror and the horizon. 



These quantities can be determined from the micrometers 

 of the mark, first, when the telescope, M, is directed on the 

 two collimators, and, second, when directed to the star ob- 

 served. In this way the observations are reduced to the 

 plane of reference, and this can be referred in the ordinary 

 way to the meridian. 



The south collimator at Rio is a mark in the focus of a 

 long-focused objective. The north collimator is the axis of 

 the prime vertical transit instrument, w^hich is itself provided 

 with collimators. Thus the meridian instruments of this ob- 

 servatory form one connected system. The device of Liais 

 serves also to investigate the flexure of the telescope M. 

 6 B, LXXXIL, February 28, 1876, 495. 



THE SOLAR ECLIPSE OP APRIL 16, 1874. 



Mr. E. J. Stone, Astronomer Royal at the Cape of Good 

 Hope, observed the total solar eclipse of April 16, 1874, at 

 Klipfontain, Africa, fifty-five miles from the coast. Mr. 

 Stone's plan of campaign was necessarily simple, as the 

 difiiculties of travel in South Africa prevented the transpor- 



