Winter halter — Personal Equation Machine. 117 



2. The apparatus should allow the error to be determined 

 in various positions of the observer. 



These two requisites are found in an apparatus arranged by 

 Professor Bakhuyzen, but the second is secured by a disposi- 

 tion, which is not convenient and which requires much time 

 for adjustment, i. e., for reflecting into the tube of the transit- 

 circle telescope (placed at a determined elevation), the image of 

 the artificial star by means of two mirrors, one of which is 

 secured to the object-end of the telescope. Of the good points 

 of the Leyden apparatus, as described by the author, I had the 

 opportunity of satisfying myself on a recent visit to that obser- 

 vatory. 



The task to be accomplished, according to the author, was 

 the fulfillment of three conditions, viz : 



1. The apparatus must be capable of application to a transit 

 or meridian circle of the larger class. 



2. It should not hinder the free movements of the telescope. 



3. The artificial star should traverse the whole field and so 

 imitate as faithfully as possible the motion of a true star. 



The design of a machine of this character was facilitated by 

 the fact that in the instrument used an artificial star was 

 already present, namely, the small luminous image which is 

 seen in telescopes with a central field-illumination, produced 

 by the little mirror attached to the inner surface of the object 

 glass. This method of illumining the field, now always used 

 in the Repsold constructions, is, therefore, the first essential to 

 the apparatus. 



The machine designed was fitted to the Cauchoix transit of 

 132 millimeters aperture, an old instrument left by the French 

 Faculty and later modernized for the Strassburg Observatory 

 by the Repsolds. The apparent motion of the artificial star 

 over the wires is secured by causing the ocular to slide 

 laterally, when the image appears to move in the direction of 

 motion of the ocular and is found in the center of its field. 

 The machine's breaks are recorded as followed : On the ocular 

 slide, insulated by a layer of caoutchouc, is a brass plate secured 

 by two scews running into sockets of hard rubber. This bears, 

 at right angles to the line of motion of the ocular, a steel 

 spring having a small brass terminal, carrying a platinum-point, 

 which touches another brass plate secured to the fixed ocular- 

 head on top of an insulating layer. With an electric wire run- 

 ning to each of the insulated plates mentioned, the current is 

 closed, when the platinum-point touches the brass underneath 

 it. In the last-named brass plate a number of parallel lines, 

 corresponding as accurately as possible to the position of the 

 reticule-wires, have been drawn by a dividing engine and filled 

 up with an insulating substance. The platinum-point in pass- 



