

1895.] The Electrical Measurement of Starlight. 143 



Take a tube, AB, of soft glass, the diameter of the bore of which 

 is 1 mm., or smaller if desired ; take a short length, AL, of aluminium 

 wire, which nearly fits the tube, and to one end, L, of this piece 

 of aluminium attach a platinum wire, LP, which emerges from 

 the end, B, of the glass tube, the contact at L being made by 

 boring a fine hole through the aluminium and pinching the two 

 metals together ; then, in the flame of a Bunsen burner or a blow- 

 pipe, melt the glass well round the aluminium, until the glass fits 

 round the aluminium as tightly as possible. The contact of the 

 glass and the aluminium should be perfect, or, at least, liquid-tight, 

 and, unfortunately, it has been found hitherto impossible to realise 

 this condition. If this condition could be attained, the photo-electric 

 cells would remain constant in their action for a very long time, if 

 not, indeed, permanently. At present, owing to this want of liquid- 

 tightness, about four weeks seems to be the limit of constancy. 

 (There is also another condition essential to constancy, which will be 

 presently mentioned.) 



The next step is to grind the end A of the tube on fine sand or 

 emery-paper until a flat surface is formed by the end of the aluminium 

 wire and the wall of the glass tube, the end of the aluminium wire 

 being then scraped clean. 



Now place the tube AB, with the end A uppermost, between two 

 nearly vertical plates of asbestos, the end A just appearing beyond 

 the edges of the plates of asbestos ; on the middle of the aluminium 

 wire at A place a very small piece of selenium (about the size of a 

 very small pin-head) ; heat the asbestos by means of a spirit lamp or 

 a Bunsen flame until the selenium melts on the end A of the tube. 

 Care must be taken to keep the flame away from the selenium, so 

 that the latter melts in virtue of the heat of the aluminium wire. 

 The selenium now lies as a very black little liquid globule on the end 

 of the tube, and it must be spread uniformly over the end of the tube 

 by means of a heated glass rod. The layer of selenium should not be 

 a thick one. The flame being removed, allow the selenium to cool 

 into a hard black layer. When it reaches this condition, apply the 

 heat again, as before, until the black surface changes into one with a 

 uniform brownish-grey colour, the heat being continued after this 

 with great care until the selenium is on the point of melting again 

 into a black liquid. On the first sign of this latter change, instantly 

 remove the heat and blow over the surface of the selenium. This 

 will at once stop the tendency to melt, and the surface will then be 

 in its most sensitive state. There should be no glossy streaks on the 

 surface ; if there are, it must be heated over again and the whole 

 process repeated. Screen the tube from light and allow it to cool 

 for a few minutes ; it will then be ready to put into the cell with 

 cenanthol. 



VOL. LVIII. M 



