316 



SCIENTIFIC AMUSEMENTS. 



refrangible rays, especially the red, orange, and yellow visible rays, 

 and the still less refrangible heat rays (vide also Chapter XXI V.). 

 Expt. 350. Combination of Colours. The preceding experi- 

 ment shows that sunlight produces on the eye a total effect due to 

 the combined action of all the prismatic colours, and this combined 

 effect is the sensation called whiteness. It should thence result 

 that if all these separate colours could be recombined in proper 

 proportions, white light should result. That this is so can be 

 readily shown by breaking up a ray of sunlight by means of a 

 prism, so as to give the prismatic colours as in the last experiment, 

 and then viewing the spectrum through a precisely similar prism 

 in an inverted position. By suitably adjusting the position of the 

 screen and the two prisms, it can be shown that, whereas all the 

 prismatic colours are in view when the screen is looked at directly, 

 when viewed through the second prism they become all blended 

 together and produce the sensation of whiteness. The same 

 result may be produced by bringing all the rays to a focus by 

 means of a concave mirror (Expt. 361) or convex lens (Expt. 365). 

 Another way of showing the same effect is to 

 mount the prism on an axle, capable of rapidly 

 revolving in such a way that each colour is thrown 

 in succession on the same portion of a screen 

 as the prism moves (fig. 158); in this way a 

 band of white light becomes visible, all the tints 

 being blended together. The combination of all 

 the prismatic tints to white may also be shown 

 by arranging a series of small mirrors instead of 

 a screen to receive the spectrum, each mirror 

 being so adjusted as to reflect the light falling 

 upon it to the same spot on a screen properly 

 placed ; this spot will then appear white. But 

 if any of the mirrors be removed or shaded by 

 an opaque screen, a coloured spot will be pro- 

 duced, inasmuch as all the tints of the spec- 

 trum are not present, and consequently the effect 

 on the eye is different from that of pure white- 

 ness (vide Expt. 354). 



Expt. 351. Newton's Colour Disc. The 

 effect of a prism in breaking up white light 

 into colours was first studied by Sir Isaac 

 Newton, to whom is also due a simple experiment illustrating the 

 production of white light by the combination of colours. A disc 

 is painted in sectors of the different prismatic colours as nearly as 

 they can be matched, and is then set in rapid rotation (conveni- 



Fig. 158. Recom- 

 position of White 

 Light. 



