64 Capt. Abney on the Measurement of the Luminositt/ and 



reflected from coloured surfaces, General Festing and myself 

 employed rotating sectors, movable when in motion, but the 

 motion was slow, being effected by means of a screw. Mr. 

 Hilger, in whose hands was placed a rough design for an 

 improved set of rotating sectors, produced an instrument in 

 which the sectors would be opened and closed rapidly by 

 means of a simple lever arrangement without any screw- 

 motion. The instrument is shown in the accompanying 

 figure. A, B are the sectors which can be opened at pleasure 

 by means of the lever, D, moving along an arc, C, on which 

 is a scale of degi-ees. The sectors are rotated by a small 

 electromotor, with sufficient speed to prevent scintillation. 



The apparatus at once got over the difficulty of getting a 



rapid alternation of intensity in any light falling on a surface 

 when the instrument was placed between it and such surface. 



