266 Dr. S. Bid well. On Negative After-images, and 



in fig. 3, on a scale of one-sixteenth. By means of the condenser B, the 

 image of the positive crater of the electric arc A is projected upon the 

 slit of the collimator D. The emergent parallel rays are refracted by 

 the prism E, and thence pass successively through a circular aperture in 

 the diaphragm F, through the achromatic lens G, and through an 

 opening in the rotating disc H (which renders the light intermittent) 

 until they reach the slit-screen I, upon the face of which the spectrum 

 is focussed by the lens G. The screen contains three adjustable 

 vertical slits, the position of which can be varied ; one, two, or three 

 selected portions of the spectrum may be allowed to pass through the 

 slits to the large lens K, which is arranged to project a sharp image of 

 the circular aperture in the diaphragm F upon the white screen L. 

 This image constitutes the " colour-patch " ; it is illuminated by a 

 uniform mixture of the spectrum-rays transmitted by the slit-screen. 



In front of the collima tor-slit D is placed a mirror C, from the back 

 of which a strip of the silver, 20 mm. long and 4 mm. wide, has been 

 removed. So much of the unabsorbed light from the electric arc as 

 does not pass through the clear glass to the collimator-slit is reflected, as 

 shown by the dotted line, through the lens M to the mirror N ; thence 

 it is again reflected through an aperture in the diaphragm (where 

 an image of the condenser B is formed by the lens M) ; it then passes 

 (intermittently) through an opening near the circumference of the 

 rotating disc H to the wooden screen P, upon which an elliptical 

 image, about 12 cm. by 4*5 cm., of the positive crater is formed. The 

 image is crossed by a dark vertical band, corresponding to the space 

 of clear glass in the mirror C. An opening in the screen P is 

 furnished with an iris-diaphragm, the aperture of which can be varied 

 from 2 mm. to 30 mm. The mirror N is so placed that a portion of 

 the image of the crater on one side or the other of the dark band may 

 cover the iris-diaphragm. A lens Q focusses an image of the aperture 

 in the iris-diaphragm upon the screen L, the disc of white light thus 

 formed being concentric with the colour-patch. 



The following are details of the apparatus : The collimator-slit is 

 adjustable by a screw having a divided head ; the achromatic lens at 

 the other end has a clear aperture of 2 -86 cm. (1J- inch) and a focal 

 length of 25 '4 cm. (10 inches). The extra dense flint-glass prism E 

 has a refracting angle of 60°, and its faces are 5*1 cm. (2 inches) 

 square. The, diameter of the circular aperture in the diaphragm F is 

 2-3 cm. ( t 9 q- inch). The focal length of the achromatic lens G is 

 76 cm. (30 inches), and its diameter 5*1 cm. (2 inches). 



The zinc disc, H, as seen from the lantern, is represented in fig. 4. 

 Its diameter is 34 cm. ; the opening near the centre extends to 45° 

 and that near the circumference to 135°; both could be varied by 

 movable zinc sectors, but the angles specified were found to be gener- 

 ally the most effective. The disc is driven by an electric motor in 



