Note 44 — CHAP. VIII. Of Light. It’s proportional quantities on the different Planets. It’s Refractions in Water and Air. The Atmosphere; it’s weight and properties. The Horizontal Moon.

Fig. V.

An Angle is the inclination of two right lines, as IH and KH, meeting in a point at H; and in describing an Angle by three letters, the middle letter always denotes the angular point: thus, the above lines IH and KH meeting each other at H, make the Angle IHK. And the point H is supposed to be the center of a Circle, the circumference of which contains 360 equal parts called degrees. A fourth part of a Circle, called a Quadrant, as GE, contains 90 degrees; and every Angle is measured by the number of degrees in the arc it cuts off; as the angle EHP is 45 degrees, the Angle EHF 33, &c: and so the Angle EHF is the same with the angle CHN, and also with the Angle AHM, because they all cut off the same arc or portion of the Quadrant EG; and so likewise the Angle EHF is greater than the Angle CHD or AHL, because it cuts off a greater arc.

The nearer an object is to the eye the bigger it appears, and under the greater Angle is it seen. To illustrate this a little, suppose an Arrow in the position IK, perpendicular to the right line HA drawn from the eye at H through the middle of the Arrow at O. It is plain that the Arrow is seen under the Angle IHK, and that HO, which is it’s distance from the eye, divides into halves both the Arrow and the Angle under which it is seen: viz. the Arrow into IO, OK, and the Angle into IHO and KHO: and this will be the case whatever distance the Arrow is placed at. Let now three Arrows, all of the same length with IK, be placed at the distances HA, HC, HE, still perpendicular to, and bisected by the right line HA; then will AB, CD, EF, be each equal to, and represent IO; and AB (the same as IO) will be seen from H under the Angle AHB; but CD (the same as IO) will be seen under the Angle CHD or AHL; and EF (the same as IO) will be seen under the Angle EHF, or CHN, or AHM. Also, EF or IO at the distance HE will appear as long as CN would at the distance HC, or as AM would at the distance HA: and CD or IO at the distance HC will appear as long as AL would at the distance HA. So that as an object approaches the eye, both it’s magnitude and the Angle under which it is seen increase; and as the object recedes, the contrary.