492 Proceedings of the Royal Irish Academy. 



a pei'pendicular axis, which, by a simple small clockwork can be 

 turned around itself. Each arm carries on the end a lamp which, by 

 the help of a screen with a small hole in it, and a lens, can produce an 

 artificial star, which, by the rotation of the axis, passes across a wire 

 represented by a veiy thin strip of black paper pasted on a piece of oil 

 paper, curved cylinchically, and opposite which a small telescope is 

 placed. In the moment one of the stars is bisected by the strip of paper, 

 an electric current is made. This is done by a copper fork, with two 

 prongs fixed at the end of the aiTa, and at the same time touching two 

 drops of quicksilver in which the conducting wii'es end. The forks 

 may be moved a little by screws, and must be adjusted carefully, 

 so that they establish the current exactly when the star is placed 

 behind the wii'e. 



The second instrument, which at present is most in use, is only a 

 modification of the first one. It has only one arm, which by clock- 

 work is drawn from one side of a mahogany board, in which one end 

 of it (that at which the lamp is placed) is fixed. The velocity of the 

 motion can be easily altered (as also in the former instniment). At 

 the free end of the axis is a lens, which causes an image of a small hole 

 in a screen before the lamp to fall on a piece of oil paper (curved as a 

 cylinder) on which any number of dark perpendicular lines may be 

 dravni. Within reach of the observer (as he stands before the tele- 

 scope) is a string, by which he, when a transit is over, can bring the 

 arm back to its original position (hereby winding up the clock again), 

 so that a new transit may take place. The metallic contact is here, as 

 in the former instrument, produced by a di'op of quicksilver, but this is 

 placed at the end of the arm, and accordingly moved along with it, 

 while the thin brass wires which successively dip themselves into the 

 drop are fixed on a brass arch, on the mahogany board. These wires 

 are leaning heavily against small levers, which may be moved a very 

 little around their centre by means of screws. In this way, each, 

 contact apparatus may be carefully adjusted. The instrument may be 

 used for either kind of transit observations; the different electric 

 currents may be arranged for the method of observing, by a commu- 

 tator, without any loss of time. 



A slight modification of this apparatus has, under the name of time- 

 collimator, been much in use on the Continent, only having a strip of 

 metal instead of the quicksilver drop.*' 



We have now given a short description of all the instruments for 

 finding personal errors which have come into practical use.f It is 



" Ueber einen neuen Apparat zur absoluten Bestimmimg von persbnliclieii FeMern 

 bei astron. Beobachtungen." Both instruments are shortly described in the 

 Annals of the Observatory in Leyden, ii., 1870, pp. 19 and foil. (Beschi-eibxmg 

 der ZeitcoUimatoren der Stemwarte in Leiden). 



* Bericht der Conferenz der Europaischen Gradmessung, Berlin, 1867. Annual 

 Eeports of the Pulko-wa ObseiTatory (Jahresberichte, e.c, 1869, page 8; 1870, 

 page 5). 



t Professor Harkness has suggested a veiy simple apparatus (Report on the 



