A NEW THEORY OF LIGHT AND COLOURS. 4 6 5 



proper for metal, whereby as I imagined, the figure also would be 

 corrected to the last; I began to try what might be effected in this 

 kind, and by degrees so far perfected an instrument (in the essential 

 parts of it like that I sent to London,) by which I could discern 

 Jupiter's four concomitants, and showed them divers times to two 

 others of my acquaintance. I could also discern the moon-like phase 

 of Venus, but not very distinctly, nor without some niceness in dispo- 

 sing the instrument. 



From that time I was interrupted till this last autumn, when I 

 made the other. And as that was sensibly better then the first 

 (especially for day objects,) so I doubt not, but they will be still 

 brought to a much greater perfection by their endeavours, who, as you 

 inform me, are taking care about it at London. 



A 



B 



Fig. 1. [Figure 13, Plate 14, in the Original.] 



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 reflective 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 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 difform 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: con- 

 cerning 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 : — 



vol. lxi. — 30. 



