And that this is true, not onely in one, but in every Ray that goes to the constitution of the Primary Iris; nay, in every Ray, that suffers only two refractions, and one reflection, by the surface of the round body, we shall presently see most evident, if we repeat the Cartesian Scheme, mentioned in the tenth Section of the eighth Schem. 6.
Fig. 3. Chapter of his Meteors, where E F K N P in the third Figure is one of the Rays of the Primary Iris, twice refracted at F and N, and once reflected at K by the surface of the Water-ball. For, first it is evident, that K F and K N are equal, because K N being the reflected part of K F they have both the same inclination on the surface K that is the angles F K T, and N K V made by the two Rays and the Tangent of K are equal, which is evident by the Laws of reflection; whence it will follow also, that K N has the same inclination on the surface N, or the Tangent of it X N that the Ray K F has to the surface F, or the Tangent of it F Y, whence it must necessarily follow, that the refractions at F and N are equal, that is, K F E and K N P are equal. Now, that the surface N is by the reflection at K made parallel to the surface at F, is evident from the principles of reflection; for reflection being nothing but an inverting of the Rays, if we re-invert the Ray K N P, and make the same inclinations below the line T K V that it has above, it will be most evident, that K H the inverse of K N will be the continuation of the line F K, and that L H I the inverse of O X is parallel to F Y. And H M the inverse of N P is Parallel to E F for the angle K H I is equal to K N O which is equal to K F Y, and the angle K H M is equal to K N P which is equal to K F E which was to be prov'd.
So that according to the above mentioned Cartesian principles there should be generated no colour at all in a Ball of Water or Glass by two refractions and one reflection, which does hold most true indeed, if the surfaces be plain, as may be experimented with any kind of prisme where the two refracting surfaces are equally inclin'd to the reflecting; but in this the Phænomena are quite otherwise.
The cause therefore of the generation of colour must not be what Des Cartes assigns, namely, a certain rotation of the Globuli ætherei, which are the particles which he supposes to constitute the Pellucid medium, But somewhat else, perhaps what we have lately supposed, and shall by and by further prosecute and explain.
But, First I shall crave leave to propound some other difficulties of his, notwithstanding exceedingly ingenious Hypothesis, which I plainly confess to me seem such; and those are,
First, if that light be (as is affirmed, Diopt. cap. 1. §. 8.) not so properly a motion, as an action or propension to motion, I cannot conceive how the eye can come to be sensible of the verticity of a Globule, which is generated in a drop of Rain, perhaps a mile off from it. For that Globule is not carry'd to the eye according to his formerly recited Principle; and if not so, I cannot conceive how it can communicate its rotation, or circular motion to the line of the Globules between the drop and the eye. It cannot be by means of every ones turning the next before him; for if so, then onely all the Globules that are in the odd places must be turned the same