perturbed by reason of the transverse reaction from A towards C, represented likewise by the short transverse lines in the triangle A H I. And in the same manner there is a double perturbation represented by the transverse lines in the triangles C G K and C K L. But as for the light between A I and C G, it will not be perturbed; because, if there were in all the points of the strait lines A B and A C the same action which is in the points A and C, then the plane of the triangle C G K would be everywhere coincident with the plane of the triangle A F H; by which means all would appear alike between A and C. Besides, it is to be observed, that all the reaction at A tends towards the illuminated parts which are between A and C, and consequently perturbeth the first light. And on the contrary, that all the reaction at C tends towards the parts without the triangle or without the prisma A B C, where there is none but second light; and that the triangle A F H shows that perturbation of light which is made in the glass itself; as the triangle A H I shows that perturbation of light which is made below the glass. In like manner, that C G K shows the perturbation of light within the glass; and C K L that which is below the glass. From whence there are four divers motions, or four different illuminations or colours, whose differences appear most manifestly to the sense in a prisma, whose base is an equilateral triangle, when the sunbeams that pass through it are received upon a white paper. For the triangle A F H appears red to the sense; the triangle A H I yellow; the triangle C G K green, and approaching to blue; and lastly, the triangle C K L appears purple. It is therefore evident that when weak but first light passeth through a more resisting diaphanous body, as glass, the beams, which fall upon it transversely, make redness; and when the same first light is stronger, as it is in the thinner medium below the strait line A C, the transverse beams make yellowness. Also when second light is strong, as it is in the triangle C G K, which is nearest to the first light, the transverse beams make greenness; and when the same second light is weaker, as in the triangle C K L, they make a purple colour. [Sidenote: Why the moon and the stars appear redder in the horizon than in the midst of the heaven.]