47o 



POPULAR SCIENCE MONTHLY. 



at a further distance of about ten or twelve feet. If at that distance 

 you intercept this light with a sheet of white paper, you will see the 

 colours converted into whiteness again by being mingled. But it is 

 requisite, that the prism and lens be placed steady, and that the paper 

 on which the colours are cast be moved to and fro ; for by such motion, 

 you will not only find at what distance the whiteness is most perfect, 

 but also see how the colours gradually convene, and vanish into white- 

 ness, and afterwards having crossed one another in that place where 

 they compound whiteness, are again dissipated and severed, and in an 

 inverted order retain the same colours which they had before they 

 entered the composition. You may also see, that if any of the colours 

 at the lens be intercepted, the whiteness will be changed into the 

 other colours. And therefore that the composition of whiteness be 

 perfect, care must be taken that none of the colours fall beside the lens. 

 In the annexed design of this experiment, ABC expresses the 

 prism set endwise to sight, fig. 14, pi. 14, close by the hole F of the 

 window EG. Its vertical angle ACB may conveniently be about 60 

 degrees: MIST designs the lens. Its breadth 2y 2 or 3 inches. SF 

 one of the straight lines, in which difform rays may be conceived to 

 flow successively from the sun. FP and FE, two of those rays un- 

 equally refracted, which the lens makes to converge towards Q, and 

 after decussation to diverge again. And HI the paper, at divers 

 distances, on which the colours are projected; which in Q constitute 

 whiteness, but are red and yellow in E, r, and p, and blue and purple 

 in P, p, and n. 



Fig. 2. Figure 14, Plate 14, of the Original. 



If you proceed further to try the impossibility of changing any 

 uncompounded colour, (which I have asserted in the 3d and 13th 

 propositions) it is requisite that the room be made very dark, least 

 any scattering light mixing with the colour disturb and allay it, 

 and render it compound, contrary to the design of the experiment. It 

 is also requisite, that there be a perfecter separation of the colours 

 than, after the manner above described, can be made by the refraction 

 of one single prism, and how to make such further separations, will 

 scarcely be difficult to them that consider the discovered laws of re- 



