

1895.] The Electrical Measurement of Starlight. 149 



of the cross-lines on gg ; mark on the glass at h with a fine brush 

 point a dot exactly opposite the intersection of the cross-lines of gg ; 

 remove gg oat of the field of view by the vertical motion of the screw 

 attached to it and to the plate DD this being done in order that the 

 light of the star may not have to pass through the glass plate on gg. 

 This completes the adjustment. 



To use the instrument with a star, keep BB stopped by the stop 1, 

 so that the mark or dot at h is in position to receive the image of 

 the star. Suppose, then, that by the adjustment of AA relatively to 

 the telescope the image of the star falls exactly on this mark. Now 

 by means of the handle H move BB until it is stopped by the stop 2. 

 This brings the centre of the sensitive surface into the position occu- 

 pied by the mark at h, i.e., the image of the star is now falling on the 

 sensitive surface of the cell. 



When we desire to throw off the star, we can do so by moving BB 

 with the handle H until it is stopped by the stop 1 ; but it is better 

 to effect this by moving the telescope itself in declination, without 

 going near the cell, until the star is out of the field, as indicated by 

 the finder ; in this latter way the cell suffers no disturbing effects of 

 temperature, &c. 



Connection of Cell with Electrometer. 



One pole (the insensitive) of the cell was connected with earth by a 

 wire attached to a gas-pipe, while the other (that of the sensitive 

 surface) was attached to a fine uncovered copper wire, carefully 

 insulated throughout, which passed down through a shellac plug 

 in the floor of the observation room to the electrometer in the room 

 below. 



The readings of the electrometer deflections caused by the light of 

 the stars were made in the lower room by Professor Fitzgerald, while 

 Mr. Wilson and I attended to matters upstairs. But in this part of 

 the work the services of my two colleagues were of very much greater 

 use than mine. 



The Observations. 



Regulus was the first star taken, on the night of the llth April, 

 and only two observations of the deflection on the electrometer scale 

 produced by its light were made before proceeding to an examina- 

 tion of Arcturus for comparison. It is thought desirable to show in 

 tabular form a few of the results obtained. A Leclanche cell pro- 

 duced on the scale a deflection of 530 mm., the scale being about 

 7 feet from the electrometer. 



Every photo-electric cell of the type previously described and of 

 maximum sensitiveness has a certain native or disturbing E.M.F., 



