LIGHT: COLOUR 



287 



made to answer, but a lens is best. A coloured image, or, with 

 a wedge-prism, two complementary images, may be produced 

 as before. 



But if a large convex lens say 6 inches diameter and 14 

 or 15 inches in focus forms part of the available apparatus, 

 it will answer just as well as a cylindrical lens. The top and 

 bottom of the lens may be screened with black card so as to 

 leave a horizontal stripe 2 inches deep, all across the centre ; 

 but even this is not necessary, as the rays from the prism will 

 only strike the centre. With this lens exactly the same ex- 

 periments may be 

 performed. 



Another method 

 is adopted by Pro- 

 fessors Eli Blake and 

 Ogden Eood, the 

 coloured rays from 

 the prism being re- 

 ceived at a distance 

 of three feet or more 

 upon a strip of 

 the thinnest silvered 

 glass procurable, 

 about 18 inches long 

 by 3 inches wide. This can be flexed by the hands so as to re- 

 unite the colours in a white reflected image ; or the plate may 

 be bent in a simple frame by a screw ; or a cylindrical mirror 

 may be employed such as is used for producing caricature 

 images. 



The next two methods depend upon persistence of vision, 

 any impression upon the retina remaining for nearly half-a- 

 second (the eighth of a second so often mentioned does not 

 nearly represent the fact with most people). The rays from 

 the prism p (fig. 153) are received on the plane-reflector B, 

 in its vertical socket, in which the mirror must move easily. 



FIG. 153. Rocking spectrum 



