368 CAPTAIN ^W. DE W. ABNEY AND DR. T. E. THORPE ON PHOTOMETRIC 
was inserted during the time of observation. As before said, the image of the Moon 
was accurately focussed on the screen inserted at S, and was viewed through the 
opening at O. The wires to the lamp passed through the slot; the carbon-resistance 
(fig. 3 ) and also the galvanometer should have been introduced into the circuit. 
Pig. 3. 
The carbon-resistance used was one supplied by Mr. Varley, and the description is 
taken from a paper already referred to. It consisted of a series of pieces of carbonised 
cloth, more or less in contact. The carbonised cloth is represented by C (fig. 3), which 
tills the whole length from A to D when loosely packed. At B is a plate to which Tg 
is attached, and which can be separated more or less from a fixed metal plate to which 
T^ is connected by the arm E, which is moved by the screw At A is an insulated 
block, carrying another plate to which Tg is attached, and A can be carried backwards 
or forwards by means of the screw S 3 . For some purposes the main current can be 
brought in at Tg, and leads be taken from T 3 and T^, thus forming part of a 
Wheatstone bridge. During the eclipse the terminals Tj and Tg were used. 
Eig. 4. 
The connections were made as shown in fig. 4. The current from 10 cells of a care¬ 
fully made up Grove battery, B, was passed through the lamp L. A shunt, including 
the galvanometer, G, and the resistance, R, was made. The brightness of the lamp 
was thus increased by adding more resistance to the shunt. Consequently, in the 
measures made, the highest readings of the galvanometer showed the lowest intensity 
of light. It would have been better had the ordinary plan of putting the galvano¬ 
meter and resistance in the main circuit been adopted, but when once the value of 
