It is well known that when a light of any kind shines through a dense medium it will appear larger than when seen through a lighter medium. This will be more remarkable when the medium holds aqueous particles in solution,--as in a damp or foggy atmosphere the light of a gas-lamp will seem greater at a given distance than it will under ordinary circumstances. In the diagram, Figure 30, it is evident that H D is less than H 1, H 3, or H 5. The latter (H 5) represents the greater amount of atmosphere which the Sun has to shine through when approaching the horizon; and as the air near the Earth is both more dense and more damp, or holds more watery particles in solution, the light of the Sun must be dilated or enlarged as well as modified in colour. But the enlarged appearance of the Sun when rising and setting is only an optical impression, as proved by actual measurement. “If the angle of the Sun or Moon be taken either with a tube or micrometer when they appear so large to the eye in the horizon, the measure is identical when they are in the meridian and appear to the eye and mind but half the size. The apparent distance of the horizon is three or four times greater than the zenith. Hence the mental mistake of horizontal size, for the angular dimensions are equal; the first 5° is apparently to the eye equal to 10° or 15° at 50° or 60° of elevation; and the first 15° fill a space to the eye equal to a third of the quadrant. This is evidently owing to the ‘habit of sight,’ for with an accurate instrument the measure of 5° near the horizon is equal to 5° in the zenith.”[10] [10] “Million of Facts,” by Sir Richard Philips, p. 537. SECTION 9. CAUSE OF SOLAR AND LUNAR ECLIPSES.