34 



A POPULAR ACCOUNT 



are characterised with such singular ele- 

 gance and ingenuity, that we shall not 

 apologise for giving the particulars of 

 them at some length. 



The prismatic spectrum being project- 

 ed on a screen, a white paper was 'held 

 before it, in such a manner as not to in- 

 tercept the rays from the prism, and so 

 that the paper should be as nearly as 

 possible equally distant from all the 

 colours. Under these circumstances, the 

 paper appeared white. The colours which 

 produced this white were evidently the 

 several colours of the spectrum reflected 

 from the screen upon the paper, and 



consequently reflected in the same pro- 

 portions as they hold in the spectrum it- 

 self ; from whence we may infer that the 

 mixture of these colours produces white. 

 If any of the colours of the spectrum be 

 intercepted, the paper will appear to be 

 illuminated with that colour which would 

 be produced by the mixture of those 

 which remain; a circumstance which 

 further confirms the inference, that the 

 white produced, when no light is inter- 

 cepted, is the consequence of the mix- 

 ture of all the colours. 



Let the spectrum fig. 35, be pro- 

 jected upon a lens M N, which will 



cause the coloured light to converge to 

 its focus G, and there to fall on white 

 paper. If the paper thus illuminated be 

 moved to and from the lens, it will be 

 found that when near the lens the paper 

 will be intensely coloured. As its dis- 

 tance from the lens is increased, the co- 

 lours will seem to approach each other, 

 and be collected into a smaller space, 

 until at last, at the focus G, they will be 

 collected and perfectly mixed together: 

 here the illuminated spot on the paper 

 will be white. By removing the paper to 

 a greater distance from the lens, the 

 rays which before converged, having 

 crossed each other at the focus G, will 

 now diverge. The colours also will be 

 inverted, those rays which were above in 

 the former case being now below, and 

 vice versa. 



Lot the paper be now placed at the 

 focus, so as to be illuminated with white 

 light free from colour. We are to prove 

 that this whiteness arises from the ad- 

 mixture of all the coloured lights of the 

 spectrum in their due proportions. Let 

 all the colours except the red be inter- 

 cepted by an opaque screen, placed be- 

 tween the prism and the lens. The spot 

 on the paper will now appear red. By 

 raising the screen let the orange be ad- 

 mitted with the red through the lens. 

 The spot on the paper will now take a 

 tint which would be produced by a mix- 

 ture of red and orange, Again, let the 



yeiiow light be admitted, and a similar 

 result will be obtained, the colour being 

 one which would be produced by the 

 mixture of red, orange, and yellow. In 

 a word, let any number of the prismatic 

 colours be intercepted between the lens 

 and the prism, and the colour on the 

 paper will be that due to the mixture of 

 those colours which are not intercepted. 

 From which we infer that if no colour 

 be intercepted, the white light on the 

 paper must arise from the mixture of all 

 the colours. 



Let XY, jig. 36, be an instrument 

 formed like a comb, with teeth about an 

 inch and a half broad, at intervals of 

 about two inches asunder. By inter- 

 posing successively the teeth of this in- 

 strument between the prism and the 

 lens, a part of the colours was inter- 

 cepted, while the rest went through be- 

 tween the teeth. The teeth of this in- 

 strument being passed before the lens, 

 all the colours are successively thrown 

 upon the paper. Now, when this mo- 

 tion is rapid, so that the colours on the 

 paper succeed each other in very quick 

 succession, the eye loses all sense of co- 

 lour, and the paper appears white. Yet 

 it is certain that the paper is not at any 

 instant white. In this case the percep- 

 tion of whiteness is produced by the 

 continuance of the impression which 

 each colour makes upon the sense of 

 sight, until all the other colours have 



