11. Besides, by the attrition or rubbing of one body against another, as of wood against wood, we find that not only a certain degree of heat, but fire itself is sometimes generated. For such motion is the reciprocation of pressure, sometimes one way, sometimes the other; and by this reciprocation whatsoever is fluid in both the pieces of wood is forced hither and thither; and consequently, to an endeavour of getting out; and at last by breaking out makes fire. [Sidenote: The distinction of light into first, second, &c.] 12. Now light is distinguished into, first, second, third, and so on infinitely. And we call that first light, which is in the first lucid body; as the sun, fire, &c.: second, that which is in such bodies, as being not transparent are illuminated by the sun; as the moon, a wall, &c.: and third, that which is in bodies not transparent, but illuminated by second light, &c. [Sidenote: The causes of the colours we see in looking through a prisma of glass, namely, of red, yellow, blue, & violet colour.] 13. _Colour_ is light, but troubled light, namely, such as is generated by perturbed motion; as shall be made manifest by the red, yellow, blue and purple, which are generated by the interposition of a diaphanous prisma, whose opposite bases are triangular, between the light and that which is enlightened. [Sidenote: [Discussion of Figure 27.2]] For let there be a prisma of glass, or of any other transparent matter which is of greater density than air; and let the triangle A B C be the base of this prisma. Also let the strait line D E be the diameter of the sun's body, having oblique position to the strait line A B; and let the sunbeams pass in the lines D A and E B C. And lastly, let the strait lines D A and E C be produced indefinitely to F and G. Seeing therefore the strait line D A, by reason of the density of the glass, is refracted towards the perpendicular; let the line refracted at the point A be the strait line A H. And again, seeing the medium below A C is thinner than that above it, the other refraction, which will be made there, will diverge from the perpendicular. Let therefore this second refracted line be A I. Also let the same be done at the point C, by making the first refracted line to be C K, and the second C L. Seeing therefore the cause of the refraction in the point A of the strait line of A B is the excess of the resistance of the medium in A B above the resistance of the air, there must of necessity be reaction from the point A towards the point B; and consequently the medium at A within the triangle A B C will have its motion troubled, that is to say, the strait motion in A F and A H will be mixed with the transverse motion between the same A F and A H, represented by the short transverse lines in the triangle A F H. Again, seeing at the point A of the strait line A C there is a second refraction from A H in A I, the motion of the medium will again be