288 OPTICAL PROJECTION 



Then a rocking motion is given, and as this motion becomes 

 more rapid, the spectrum becomes white over all the middle 

 portion. If the hand is unable to rock the reflector .with 

 sufficient rapidity, a stand may be employed which will im- 

 part the motion mechanically by a short arm from a multi- 

 plying wheel. 



A most instructive experiment is that so well known as 

 ' Newton's disc.' A circular card which can be rapidly 

 rotated on its centre is painted in sectors with the principal 

 hues of the spectrum in their due proportions, either in one 

 sector of each colour, or several sets, fig. 154 showing the 

 disc painted in four sets. This is arranged facing the lantern, 

 and all the light from the nozzle is made to just cover the 



disc and no more, the 

 lantern being so placed 

 that the audience can 

 see the face of the disc. 

 When this is rapidly 

 rotated, it appears 

 white. 1 Newton's disc 



no. 154 FrG.i55 can also be obtained as 



a transparent slide for 



the ordinary stage of the lantern, the cost of the latter arrange- 

 ment with the wheelwork being about 12s. Qd. 



If now sectors of black paper are fastened, with drawing- 

 pins, on the card disc, or gum on the glass, so as to cover up 

 any of the colours, by so suppressing these again we get 

 colour. Or we may rotate just in front of the nozzle of the 



1 If the colours are not properly painted, of course the white will not be 

 perfect, and a disc should always be tested before purchase, or at least before 

 use. But a beautiful white can be got by this method, and it is the brighter 

 the more brilliant the colours. It is true the disc often appears a poor grey 

 as usually shown, by the general light of the room ; since each portion of it can 

 only reflect at most about one-eighth of the spectrum ; hence the grey, which 

 is merely a deficiency of light. But by keeping the room dark, and concen- 

 trating a bright light on the disc, with a good one the white is all that can be 

 desired, 



