their Relation to certain other Visual Phenomena. 267 



circuit with a variable resistance, the latter being adjusted so that the 

 speed of rotation may be a little higher than is required for the ex- 

 periment ; a short-circuit key within reach of the observer's hand 

 enables him to vary the speed at will or to keep it sensibly constant. 

 A wire attached at right angles to the axis of the disc taps a strip of 

 card at every revolution, producing a succession of audible clicks, which 

 can, when desired, be compared with the taps of a metronome beating 

 seconds. The most usual speed is from five to six turns per second. 

 The disc apparatus is supported at such a height from the table that 

 when the disc is turning in the direction of the arrow the spectrum 

 projected upon the screen I (fig. 3) is eclipsed at the moment when 

 the iris-diaphragm in the screen P is beginning to be exposed to the 

 white light. During about one-half of a revolution both the diaphragm 

 and the slits are shielded by the disc. The width of the spectrum 

 projected upon the slit-screen I (fig. 3) is 2*9 cm., and its visible 

 length in a dimly lighted room about 7 cm. ; the measured distance 

 between A 6870 (Fraunhofer line B) and A 4115 (iron line between 

 r/ and H) was approximately 6'1 cm. 



Fig. 4. Fift. 5. 





















1 









SI 















The slit-screen is shown diagrammatically in fig. 5. It consists of 

 a mahogany board, having cut in it an oblong window, 10 -4 cm. by 

 2-7 cm., over which the three brass slit-frames slide between grooved 

 guides above and below. Each slit-frame is 1:8 cm. wide, and has an 

 aperture of 2*5 cm. by 0'6 cm. The slit-jaws (not shown in the diagram) 

 are attached to the front surfaces of the brass frames, and are adjustable 

 in the parallel-ruler fashion, one of every pair being fixed to its frame ; 

 the slits can be opened to 0*55 cm. The two outermost slit-frames are 

 attached by screws to sliding shutters, which serve to cover such por- 

 tions of the window right and left of the slit-frames as would other- 

 wise be open to the light. The spaces between the middle slit-frame 

 and the two outer ones are closed by opaque black ribbons (shaded in 

 the diagram), constituting miniature spring-roller blinds. The axes of 

 the spring rollers are so placed (perpendicularly behind one edge of a 



VOL. LXVIII. u 



