Dreyer — On Astronomical Transit Observations. 491 



again, and pass the wires a second time, this time registering its tran- 

 sits by the interruption of the current. By placing a prism before the 

 eye-piece during this second transit, the motion of the star -will, to the 

 observer, seem to take place in the same direction as during the first 

 one. By this arrangement of the obseryations, small imperfections in 

 the contact apparatus (which would establish or interrupt the current 

 a little before or after the right moment) will have no influence at all 

 on the mean of two consecutive transits, and the bending of the 

 spring will only cause an imperceptible fault in the same. "Wolf's 

 detenninations of his personal error were always founded on 40 tran- 

 sits, 20 in each direction, and as the instrument was very carefully 

 treated, and all som'ces of faults examined, his results deserve the 

 highest confidence. 



The instruments we hitherto have mentioned have not been gene- 

 rally used by astronomers, but only by their inventors and a few other 

 persons. This is not the case with the different instruments successively 

 constructed by the late Professor Kaiser, in Leyden, which are well 

 known in the scientific world, especially the latest constructed, which 

 has often been used in determiaations of longitude on the Continent, as 

 "well as in Pulkowa, and which is generally termed a time-coUimator. 

 Kaiser has in the course of years constructed three apparatus. As 

 early as in 1851, this eminent astronomer proposed to apply the prin- 

 ciple of the nonius to determinations of time, and promised later to 

 describe an instrument based on this principle, and suitable for finding- 

 absolute personal errors.*^ This promise he carried into effect in 1863 

 by publishing a paper which, besides the description of the apparatus, 

 contains a great many observations taken with it.f The arm which 

 carried the artificial star interrupted a current in the moment of the 

 transit, which caused an electro-magnet to let its armature fall, whereby 

 a pendulum, which hitherto had been kept in its greatest elongation 

 from the vertical position, was set going. By the coincidences of tliis 

 pendulum with the clock used for the observation, the true moment of 

 the transit could bo determined with great accuracy. JN^umerous 

 experiments were made with this instrument by the astronomers in 

 Leyden, and Kaiser introduced now the custom to let astronomical 

 students practise with observations of artificial stars. 



The two other instruments of Kaiser are in principle more like 

 C. Wolf's apparatus, but may be used for chronographic as well as for 

 eye-and-ear observations.! The fii-st of them has several arms fixed on 



* Tijdsclirift voor de Wis - en Natiirkunclige Wetenschappen, xv., page 9. 



t "De volledige bepaling van personlijke fouten bij sterrekundige ^aamem- 

 ingen," in tlie xv. vol., page 173, of the Verslagen en Mededelingen der K. 

 Akademie van "Wetenschappen, AfdeUng Natixrkunde (Amsterdam, 1863). As it 

 was found that this paper was very little kno^v^l on account of the language, the 

 apparatus alone was again described in the Archives Neerlandaises des Sciences 

 exactes et naturelles, vol. i., Hague, 1866. 



X The first of them, as well as observations made with it, are described in the 

 "Verslagen en Mededelingen e. c," ii. series, vol. ii., pp. 216 and foil. (1868) : 



R.I. A. PROC, SER. II., VOL. II., SCIENCE. 3 E 



