144 Mr. G. M. Minchin. [May 1(5, 



The cenanthol cell is a small glass tube, represented in fig. 2. 

 It consists of a glass tube about 3 cm. long, and nearly a centimetre 

 in diameter, with two short glass tubes fitted into it on opposite sides ; 

 one of these is ground flat, and has a thin quartz window, QQ, 

 cemented to it with gelatine and acetic acid, or glue and glycerine, 

 or any cement tha,t will withstand the action of cenanthol ; the other 

 is tightly closed by a cork, CO, through which passes the glass tube, 

 AB, which contains the aluminium and platinum wires above 



FIG. 2. 



described. The cell is closed at one end by a ground glass stopper, 

 S, and through the other end passes a platinum wire, P', sealed in. 

 The two poles of the cell are the platinum wires P and P'. 



The light of a star is destined to shine through the quartz window 

 QQ on the centre of the sensitive surface A, which is placed in focus 

 of a telescope, or rather a little behind this focus, so that the light of 

 the star may cover the whole of the selenium area. 



The covering of the whole area A by the light is essential, for the 

 following reason : 



The seat of the electromotive force is the surface of contact of the 

 liquid and the selenium, the selenium receiving a positive and the 

 liqnid a negative charge. If, now, P is connected with one pole of 

 an electrometer and P' with the other, and if there is any portion of 

 the selenium surface which is not exposed to the light (and conse- 

 quently not the seat of an E.M. B\), this inert portion will act simply 

 as a conductor conveying a portion of the positive charge to the 

 wrong pole of the electrometer, and thus giving a diminished effect. 



The truth of this is easily verified with any kind of photo-electric 

 cell, e.g., one formed of a sensitive tinfoil surface divided into two 



