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element, at once abundant and invaluable; fuch is her libe rality to the fons of men!
On the productions of EARTH we fubfift; we ourselves are among its productions; and however endowed with reason or genius, with acuteness of thought, or depth of understanding, we trace our origin from the dust, and acknowledge our compofition is but clay. A reflection at once exalting and humiliating. Of this Earth is capable! Is this capacity attached to earth? has earth been animated, enlivened, ennobled, in the rank of Being? Is this nobility, this life, this animation, combined merely with earth! which shall foon return to its firft principles, to its native infenfibility: how amazing is this!
Equally neceffary the fervices of FIRE, equally numerous, equally valuable, whofe principles, indeed, are concealed in obfcurity; from whom we receive many, many benefits, yet whose mode of action defies our strictest scrutiny, our acuteft penetration; fecluded from detection by danger, and ofcaping from acquaintance by tenuity.
Hail, holy LIGHT, offspring of heaven! first born,
May we express thee unblamed? fince GOD is light,
Dwelt from eternity; dwelt then in thee,
Bright effluence of bright effence increate!
Whose fountain who shall tell? before the fun,
After these noble lines, what can I add ? 'tis little to repeat
the properties of light as known by mortals, little to suggest the fervices of light as received by mortals, little to survey the extent of light as comprehended by mortals, yet this is all which I can do. Summon your own powers LADIES and GENTLEMEN, trace this bright fluid through diftant realms, accompany it to other spheres, prefs on wherever it extends, follow to its termination, difcover its boundaries, examine its limits, and only ceafe where it ceafes; is this too much, too much for the powers and capacities of mortals? let us glory in the idea of what hereafter we may accomplish, and at present join the poet's prayer,
So much the rather, thou CELESTIAL LIGHT,
OBSERVATIONS ON THE PLATES
BELONGING TO LECTURE XIV.
PLATE I. REFRACTION. This figure fshews 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 denfer 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 reafon 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 defcribed 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 refraction 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 11 to 17; or nearly as 2 to 3 (lofing one third of its length). Into a diamond greater ftill.
This figure may be reversed 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 course being D B, its latter course B A; fo that inftead 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 part of such 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 supposed surface, from which a portion of them is reflected, which renders this surface visible 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 sur
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 same vifible to an eye before the body. The fecond reflection is always more confiderable than the first.
OPACITY. The pores in the former figure were supposed fimilar, 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 surface of this body (as R), it has alfo to vary its course for a fecond fet of pores, then for a third, &c. till it is totally ftifled and disappointed in its endeavours to find a paffage.