NEWTON'S DISC EXAMINED BY A PRISM. 543 



the colours under examination rhould be rendered 

 indistinct by the interference of the images of the 

 various objects around, which by the refractive effects 

 of the prism may possibly be superposed upon the 

 colours, each colour should be surrounded by a dark 

 space. A slit, 25 or 30 mm long and 3 or 4 mm broad, is 

 cut in the middle of a square piece of thin cardboard of 

 quarto size ; the cardboard and especially the inner 

 edges of the slit are painted black by a mixture of 

 lampblack and a little glue. When the paint is dry, 

 any single colour of the disc is applied close to the slit, 

 and whilst holding both together in the left hand, the 

 arm is stretched out, and the colour viewed through 

 the sulphide of carbon prism, care being taken to turn 

 the back to the window in order to have as much light 

 as possible thrown upon the slit not direct sunlight, 

 but diffused daylight. The image of the slit will, of 

 course, have to be looked for towards the left, if the 

 refracting edge of the prism is on the left side of 

 the observer. This edge as well as the slit must be 

 vertical. 



Each colour will be shown by the prism to be more 

 or less compounded of others. If the Blue is behind 

 the slit it gives out a spectrum in which only the 

 Yellow is wanting. The Red contains, besides Eed 

 and a little Orange, some traces of Green and Violet ; 

 the Green contains only the portion of the spectrum 

 between Yellowish-green and Bluish-green, besides a 

 faint Red. Hence the Red and Green are comparatively 

 more simple than the other colours. 



If, instead of the colours of the disc, other coloured 

 bodies be placed behind the slit and examined by the 



