PHYSICAL AND LITERARY. 69 



ideas of yellow and blue ; the idea of white, 

 a mixture of the ideas of all the colours ; and, 

 in general, the ideas of all compound colours; 

 a mixture of the ideas of their conftituents ? 

 In the experiments which Sir Ifaac Newton 

 performed with the toothed inftrument, the 

 component colours are not, indeed, prefented 

 to the eye all at once ; yet they follow one 

 another in fo rapid a fucceffion, that their 

 refpeftive impreflions remain in the eye till 

 they are renewed, and therefore they muft 

 affedt the mind all at once*. If a piece of 



paper 



* It is in this manner that philofophers explain {Neicf. 

 (Opt. Quen i6.) the appearance of a fiery circle, which is 

 made by a burning body whirled about fvviftly. We fliall 

 here give an account of fome other phanomena that flow froni 

 the fame principle. 



If a white rod be moved rapidly backwards and forwards 

 with an angular motion, the whole circular fpace which it 

 runs over will appear whitifli j but not equally fo, being 

 fainteft and mod dilute in the middle, and brighter towards 

 the two fides, which feem to be diftinftly terminated with 

 two white rods interfeding each qther in the center of ro- 

 tation. (See Tab. iii. Fig 7.) 



The total impreflion made upon the eye by equal fmall 

 parts of the fecSlor muft be, as the quantity of light emitted 

 from it and the frequency of the returns of the rod to it ; 

 /, e. inverfely, as the time between the returns of the rod. 



Let 



