OBSERVATIONS ON THE PLATES

BELONGING TO LECTURE XIV.

PLATE I. REFRACTION. This figure shews the attractive effect of different mediums on rays of light. If A B, be a ray fuppofed paffing in the air in one uniform direction, when arrived exceeding near the point B, it will feel the influence of the medium 3 B 4; and will be drawn by it out of that courfe it would follow (as A B C) were no fuch denser medium prefent; and will now be inclined toward the perpendicular(1 5 B 8 2), and describe the line B 7 D: varying its paffage the distance between C and D. The reason of this is, that the force of the attraction of this medium (and all others), being perpendicular to its furface, draws the ray as near as it can to its own direction, i. e. towards the perpendicular.

If around B, as a centre, be described a circle, as 1 2 3 4, and where that circle cuts the incident ray A B, and the refracted ray B D, parallel lines be drawn to the perpendi cular, as 5 6, and 7 8, the proportion of these lines to each other will inform us of the quantity and force of the different refractive powers of the two mediums. If the refrac tion be out of air into water, the line 7 8 will be to the line

56, as 3 to 4 (i. e. fhorter by a quarter of its length). If out of air into glass, it will be as II to 17; or nearly as 2 to 3 (lofing one third of its length). Into a diamond greater ftill.

This figure may be reverfed in its idea; and we may suppose the ray of light paffing from a denser medium into a rarer; where, instead of being drawn toward the perpendicular, it is deflected toward the horizon; its first courfe being D B, its latter course B A; so that instead of going on to E, its paffage is now varied the distance from E to A. It is clear that all circumstances are reverfed from what they were before, and were any one defirous to look along the line B D (fuppofing it a stick for instance), he must not place his eye at E, but at A. It is clear alfo that the It is clear alfo that the part of fuch stick, which is beneath the water, will lofe in appearance one quarter of its real length; fuch being the proportion of the line 7 8, to the line 5 6. We see also the reason why A not only fees to C (as in air he would do), but also much deeper in the fluid as B 7 D.

TRANSPARENCY. In this figure, L is a pencil of rays ftriking a fuppofed furface, from which a portion of them is reflected, which renders this furface vifible to an eye placed at a corresponding angle: this is the first reflection (R 1). But the remainder of these rays, having accommodated themselves to that direction imposed on them by the attractive power of this body, pass through it; and at its further fur

face are again divided, part paffing in to whatever medium may be behind the body, whereby the internal nature of the body becomes visible to an eye beyond it; and part forming a fecond reflection (R 2); whereby it renders the fame vifible to an eye before the body. The fecond reflection is always more confiderable than the firft.

OPACITY. The pores in the former figure were supposed similar, uniform, and alike, fo that having fuftained one inflection, the light had no impediment to its further progrefs; but in this figure, they are fuppofed to be irregular in their positions, their forms, their fizes, &c. So that after having accommodated itself to what may be at the furface of this body (as R), it has alfo to vary its course for a fecond set of pores, then for a third, &c. till it is totally ftifled and disappointed in its endeavours to find a paffage.