VOL. VI.] PHILOSOPHICAL TRANSACTIONS. (583 



I have sometimes thought to make a microscope, which in like manner 

 should have, instead of an object glass, a reflecting piece of metal. And this 

 I hope they will also take into consideration. For those instruments seem as 

 capable of improvement as telescopes, and perhaps more, because but one re- 

 flective piece of metal is requisite in them, as you may perceive by the diagram, 

 (fig. 13, pi. 14,) where AB represents the object metal, CD the eye glass, F 

 their common focus, and O the other focus of the metal, in which the object 

 is placed. 



But to return from this digression, I told you, that light is not similar, or 

 homogeneal, but consists of diftbrm rays, some of which are more refrangible 

 than others : so that of those, which are alike incident on the same medium, 

 some shall be more refracted than others, and that not by any virtue of the 

 glass, or other external cause, but from a predisposition, which every particular 

 ray has to suffer a particular degree of refraction. 



I shall now proceed to acquaint you with another more notable difformity in its 

 rays, wherein the origin of colours is unfolded : concerning which I shall lay 

 down the doctrine first, and then, for its examination, give you an instance or 

 two of the experiments, as a specimen of the rest. — ^The doctrine you will find 

 comprehended and illustrated in the following propositions : — 



1 . As the rays of light differ in degrees of refrangibility, so they also differ in 

 their disposition to exhibit this or that particular colour. Colours are not qualifi- 

 cations of light, derived from refractions, or reflections of natural bodies (as it is 

 generally believed,) but original and connate properties, which in divers rays are 

 diverse. Some rays are disposed to exhibit a red colour, and no other ; some a 

 yellow, and no other; some a green, and no other, and so of the rest. Nor 

 are there only rays proper and particular to the more eminent colours, but even 

 to all their intermediate gradations. 



2. To the same degree of refrangibility ever belongs the same colour, and to 

 the same colour ever belongs the same degree of refrangibility. The least re- 

 frangible rays are all disposed to exhibit a red colour, and contrarily, those rays 

 which are disposed to exhibit a red colour, are all the least refrangible : so the 

 most refrangible rays are all disposed to exhibit a deep violet-colour, and con- 

 trarily, those which are apt to exhibit such a violet colour, are all the most 

 refrangible. And so to all the intermediate colours, in a continued series, be- 

 long intermediate degrees of refrangibility. And this analogy betwixt colours, 

 and refrangibility, is very precise and strict ; the rays always either exactly 

 agreeing in both, or proportionally disagreeing in both. 



3. The species of colour, and degree of refrangibility proper to any particular 

 sort of rays, is not mutable by refraction, nor by reflection from natural bodie?, 



4 R 2 



