250 Captain Abney and Major-General Festing. [Dec. 8, 



sleeve C. A stud in this last engages with a screw thread of 

 long pitch cut on the axis. A horizontal movement of C thus causes 

 it, as well as the sectors S l5 to rotate with reference to the axis and 

 the other sectors (S), and therefore alters the aperture. This move- 

 ment is given by means of a vertical lever engaging with the groove 

 K on C, and which is actuated by the screw B. The aperture can 

 thus be varied between 0° and 180°, whether the instrument be in 

 motion or at rest. The instrument has been described at length, as 

 it varies in some particulars from previous ones, having been made 

 under our own supervision. The edge of S is graduated into degrees 

 so that the amount of aperture is at once known. 



A certain current is passed through L n , which is noted, and the 

 sectors opened or closed till the shadows cast by the rod appear 

 equally luminous. The motor is stopped and the aperture read off. 

 Three or four readings for each current passing through L n are 

 taken, and then the current is altered, and a new set of three or four 

 readings made. The current is altered so that the light from L n 

 varies from extreme brightness to a dull red. 



By the other plan the sectors shown in fig. 2 are detached from the 

 motor, and card disks placed on A. A resistance, R, fig. 3, has now 



Fig. 3. 



to vary to bring the light equal to the standard light diminished by 

 the disks rotating between the lamp L x and the screen with known 

 apertures. For the resistance at first we used a non-conducting 

 tube, in which about forty spherical pellets of hard carbon were 

 inserted. At one end of the latter was a brass plate to which 

 one terminal of the battery was attached, and at the other a screw 

 was inserted, which was attached to the other terminal of the 

 battery through the lamp. This screw pressed the pellets together 

 to any required degree, diminishing the resistance or increasing 

 it as occasion inquired. This answered fairly well, but not so 

 well as would be desired, as the response to the screw was some- 



