304: Messrs. H. E. Ives and E. F. Kingsbury on the 



photometer. In the former case the disk need be merely 

 opaque, in the latter case it must have carefully bevelled 

 edges and be evenly covered with magnesium oxide. A 

 series of disks having various openings were provided, several 



Fig-. 5. 



Lj, L 2 , light sources. 

 T, photometer track, 

 F, white surface. 

 D, sector disk. 



Plan of Experimental Apparatus. 



M, motor to drive disks. 

 S, electric speed-counter, 

 E, observing- aperture. 

 Nj,, N £ , Nicol prisms. 



being arranged for use with the flicker-photometer arrange- 

 ment, the majority, however, being simple thin opaque disks 

 of metal not whitened. These latter were used exclusively 

 in the critical frequency experiments. M is the electric 

 motor used to drive the disk. Its speed is regulated by a 

 series resistance and read upon an electric speed-counter S. 

 The latter consists of a small toy motor (provided with dis- 

 proportionately large and close-fitting brushes) used as a 

 generator, its E.M.F. being measured on a millivoltmeter. 

 This speed-counter was calibrated by stop-watch and revo- 

 lution-counter and found to give an accurately rectilinear 

 speed-voltage relation. E is the observing aperture, of 

 one square millimetre area. N\ and N 2 are Nicol prisms, 

 by means of which the brightness of the observed field can 

 be varied over a wide range. The effective area of the field 

 of view has a diameter of about 9^ degrees. 



