fi 490 Proceedings of the Royal Irish Academy. 



meter-screw, so that it is interrupted exactly in the moment when th& 

 pendulum is vertical, and the star in reality behind the wire. 



The simple construction of this instrument guarantees the non- 

 existence of constant errors in the results obtained by it.* But it is 

 a fault in it, that a very considerable time is necessary for taking any 

 great number of observations, as it has to be stopped, read off, and 

 set going again after every single wire-transit. 



A more suitable apparatus was invented by M. C. "Wolf, of the 

 Paris Observatory, who has described it, as well as a great number of 

 observations and special researches on the personal errors and their 

 origin, in the " Annales de I'Observatoire de Paris," Memoires, T. 

 VIII. f The artificial star is produced by a small opening in a board 

 (illuminated from behind), whose image, by a system of lenses, is 

 thrown on the plane of five wires in a small telescope. The board is 

 at one end of an arm, which by a clockwork can be made turn round 

 its centre. In this centre of the rotation is a lens of very short focal 

 length; the image of the star produced by this lens is seen through 

 another lens placed before the object-glass of the telescope. The 

 image of the star, seen by the observer in the plane of the wires, is in 

 this way made to move very slowly, so that the board moves 1 6 centi- 

 meters, while the image of the star only goes from the first to the 

 fifth wire or 12 millimetres. The transits are observed by eye and 

 ear, while the small " carriage " at the end of the arm, underneath 

 ii the board, is furnished with a contact apparatus which automatically 



I registers the transits on a strip of paper on which, also, the second- 



beats of the clock are marked. This apparatus consists of a steel 

 spring (fixed on the carriage) with a very small ball or knot at the 

 end, which is dragged along the surface of a wooden board, in which, 

 at equal distances, five thin copper strips are inlaid. The latter must, 

 before the beginning of the observations, be adjusted by fine mi- 

 crometer screws, so that the star will be behind a wire in the tele- 

 scope when the contact of the small ball with any of the copper strips 

 closes an electric current, or, during a retrograde motion of the star, 

 opens it. In either case the absolute moment of the transit will, 

 therefore, be registered on the same paper on which (by another cur- 

 rent) the seconds are marked, so that a comparison of the marks on 

 this paper with the moments of transits estimated by the observer, 

 gives the value of the personal error. 



The observation is made in the following way : — The apparatus is 

 adjusted, and the observer sets the clockwork going. The contact 

 apparatus and the star will, however, stand still till a small weight is 

 put on a plate. This causes the transit to take place. When it is 

 over, the weight is put on another plate, and the star will now go back 



* The very important researches for whicli this apparatus has heen used are, 

 ho-R-ever, independent of constant errors, as we shall see further on. 



fEecherches Bur r equation personelle dans les observations de passage. Par 

 M. C. Wolf. 



