COMMON THINGS COLOUR. 



coloured lights exhibited in the spectrum, it might be expected 

 that, these coloured lights being mixed together in the proportion 

 in which they are found in the spectrum, white light would be 

 reproduced. This is accordingly found to be the case. If the 

 spectrum formed by the prism ABC (fig. 8.) instead of being 

 thrown upon a screen, be received upon a concave reflector M N, 



Fiff. 8. 



the rays which diverged from the prism and formed the spec- 

 trum will be reflected converging to the focus r ; and after 

 intersecting each other at that point, they will again diverge, 

 the ray R r passing in the direction F n', and v F in the 

 direction r y'. 



Now, if a screen be held between F and the reflector, the 

 spectrum will be seen upon the screen. If the screen be then, 

 moved from the reflector towards the focus F, the spectrum upon 

 the screen will gradually diminish in length, the extreme colours 

 11 and v approaching each other. When it comes so near to F 

 that the extreme limits of the red and violet touch each other, 

 the central point of the spectrum will become white ; and when 

 the screen arrives at the point F, the coloured rays being all 

 mingled together, the spectrum will be reduced to a white 

 colourless spot. 



Just before the screen arrives at F, it will present the appear- 

 ance of a white spot, fringed at the top with the colours forming 

 the upper end of the spectrum, violet, blue, and green, and at the 



