.Sir Isaac Newton’s Book III.

m

that those rays which come to the surface in one line, will gooff also in one line without any parting from one another. Ac-cordingly in the attempt he succeeded so well, that a shortone, not much exceeding fix inches in length, equalled an or-dinary telescope whole length was four feet. Instruments ofthis kind to greater lengths, have of late been made, whichfully answer expectation \

Chap. V.

Of the RAINBOW.

I S h a l l now explain the rainbow. The manner of itsproduction was understood, in the general, before SirIsaacNewton had discovered his theory of colours; butwhat caused the diversity of colours in it could not then beknown, which obliges him to explain this appearance parti-cularly ; whom we shall imitate as follows. The first per-son, who expressly shewed the rainbow to be formed by thereflection of the sun-beams from drops of falling rain,was Antonio de Do minis. But this was afterwards

more fully and distinctly explained by Des Cartes.

a. There appears most frequently two rainbows; bothof which are caused by the soresaid reflection of the sun-beams from the drops of falling rain, but are not pro-duced by all the light which falls upon and are reflectedfrom the drops. The inner bow is produced by thoserays only which enter the drop, and at their entrance areso refracted as to unite into a point, as it were, upon the far-ther surface of the drop, as is represented in fig. 162 ;where the contiguous rays ah y c d, ef y coming from the

* Philos. Trans. No. 378. siin,