VOL. VII.J PHILOSOPHICAL TRANSACTIONS. 23 



was compounded of those which emerge : but if the refractions be uniform, and 

 the green persist without any change of colour, then is it original and uncom- 

 pounded. And the reason why I call it so, is, because a green indued with 

 such properties cannot be produced by any mixing of other colours. 



Now, if two green objects may to the naked eye appear of the same colour, 

 and yet one of them through a prism seem confused and variegated with other 

 colours at the edges, and the other distinct and entirely green ; or, if there may 

 be two beams of light, which flilling on a white wall, do to the naked eye exhi- 

 bit the same green colour, and yet one of them, when transmitted through a 

 prism, be uniformly and regularly refracted, and retain its colour unchanged, 

 and the other be irregularly refracted and made to divaricate into a multitude of 

 other colours ; I suppose these two greens will in both cases be granted of a 

 different origin and constitution. And if by mixing colours, a green cannot be 

 compounded with the properties of the unchangeable green, I think I may call 

 that an uncompounded colour, especially since its rays are alike refrangible, and 

 uniform in all respects. 



The same rule is to be observed in examining, whether red, orange, yellow, 

 blue, or any other colour, be compounded or not. And, by the way, since all 

 white objects through the prism appear confused, and terminated with colours, 

 whiteness must according to this distinction, be ever compounded, and that the 

 most of all colours, because it is the most confused and changed by re- 

 fractions. 



From hence I may take occasion to communicate a way for the improvement 

 of microscopes by refraction. The way is, by illuminating the object in a 

 darkened room with light of any convenient colour not too much compounded: 

 for by that means the microscope will, with distinctness, bear a deeper charge 

 and larger aperture, especially if its construction be such as I may hereafter de- 

 scribe; for the advantage in ordinary microscopes will not be so sensible. 



There remains now the third quaere to be considered, which is, whether 

 whiteness be an uniform colour, or a dissimilar mixture of all colours ? The ex- 

 periment which I brought to decide it, the animadversor thinks may be other- 

 wise explained, and so concludes nothing. But he might easily have satisfied 

 himself by trying what would be the result of a mixture of all colours. And 

 that very experiment might have satisfied him, if he had pleased to examine it 

 by the various circumstances. One circumstance I there declared, of which I 

 see no notice taken ; and it is, that if any colour at the lens be intercepted, the 

 whiteness will be changed into the other colours: if all the colours but red be 

 intercepted, that red alone, in the concourse or crossing of the rays, will not 

 constitute whiteness, but continues as much red as before ; and so of the other 



