perfe their light through the vaft expanfe of the univerfe, but the smallest spark of a lucid body must do the fame, even the finalleft globule ftruck from a fteel by a flint.

Sir Ifaac Newton obferves, that bodies and light act mutually upon each other; bodies on light, in emitting, reflecting, refracting, and inflecting it; and light on bodies, by heating them, and putting their parts into a vibrating motion, in which heat principally confifts. For all fixed bodies, he obferves, when heated beyond a certain degree, emit light, and fhine; which fhining, &c. appears to be owing to the vibrating motions of their parts; and all bodies, abounding in earthly and fulphureous particles, if fufficiently agitated, emit light, which way foever that agitation be effected. Thus, feawater fhines in a form; quick filver, when fhaken in vacuo; cats, or horfes, when rubbed in the dark; and wood, fiíh, and flesh when putrefying.

Light proceeding from putrefcent animal and vegetable fubftances, as well as from glowworms, is mentioned by Ariftotle. Bartholin mentions four kinds of luminous infects, two with wings, and two without; but it is aflerted by travellers, that in hot climates, they are found in much greater numbers, and of different fpecies. In particular, on the river Menam, which runs through Siam, a vaft number of thofe infects, called fire-flies, make a beautiful appearance in the night:

On Menam's orient ftream, that nightly fhines
With infect-lamps.

THOMSON,

There is one property of light, called refraction, which I will mention here, because it is capable of a very eafy and familiar illuftration, and will account for a very common, but feemingly extraordinary phenomenon. When a ray of light paffes out of one medium into another, it is refracted, or turned

out of its first course, according as it falls more or lefs obliquely on the refracting furface which divides the two mediums. Any perfon may exemplify this by the following experiment: put a fhilling in an empty bafin, and retire to fuch a distance, that the edge of the bafin fhall juft hide it from your fight;' then keeping yourself steady, let another perfon fill the ⚫ veffel gently with water; and, as the water rifes toward the top, the fhilling will become more and more vifible, till, at length, the whole of it will be diftinctly feen, appearing as if it had been raised above the bottom of the bafin.

This proves, that the rays of light are refracted, or bent downward, in their paffage out of the water into the air; and as they now come to the eye in a more oblique direction, the object muft neceffarily appear to be elevated, and in a different fituation from that in which it was really placed. The fame thing may also be fhown thus: place the bafin in fuch a manner, that the fun may fhine obliquely on it, and obferve where the fhadow of the rim falls upon the bottom; then fill it with water, and the fhadow will not extend fo far as it did when the veffel was empty; which fhows that the rays have changed their direction, by paffing out of one medium into another of a different density.

The lefs obliquely the rays fall, the lefs they will be refracted; and if they fall perpendicularly, they will not be refracted at all. For, in the laft experiment, the higher the fun rifes, the lefs will be the difference between the places, where the edge of the fhadow falls, in the empty and full bafin. And if a ftick be laid acrofs the bafin, and the fun's rays be reflected perpendicularly into it from a lookingglafs, the fhadow of the ftick will fall upon the fame part of the bottom, whether the bafin be full or empty. The fame effects will alfo take place, when the experiment is performed with any other fluid:

but the denfer the medium, the more will light be refracted in paffing through it.

From thefe obfervations it will readily appear, that objects can feldom be feen in their true places. We are deceived by every thing around us: the fight is no lefs fubject to error than the rest of our fenfes they all contribute to our pleasure, and promote our happiness by various means. In confe

quence of this property of refraction, we enjoy the fight of the fun when he is really below the horizon, for a little time before his rifing and for a little time after his fetting. This is alfo the cause which produces the crepufculum, or morning and evening twilight for the rays of the fun, in falling upon the higher part of the atmosphere, are reflected back to our eyes, and form a faint light, which gradually augments till it becomes day. It is in thofe brilliant colours which paint the clouds, before the rifing of the fun, that the poets have placed Aurora, or the goddess of the morn: fhe opens the gates of day with her rofy fingers; and the daughter of the Air and of the Sun has her throne in the atmosphere.

Had no fuch atmosphere exifted, the rays of light would have come to us in ftraight lines, and the appearance and difappearance of the fun would have been inftantaneous; we fhould have had a fudden tranfition from the brightest sunshine to the moft profound darknefs, and from thick darkness to a blaze of light. Refraction, therefore, is extremely ufeful, not only as it prepares us gradually for the light of the fun, but as it occafions twilight, and thus prolongs the duration of the day. Nature has eftablished thefe gradations, to heighten our pleafures by variety: the fcene is perpetually changing, but the order of things is immutable.

With what exalted fentiments of devotion ought the Contemplative Philofopher to confider the va rious and unfpeakably beautiful phenomenons of

light! We find it to be little lefs than the life and pleasure of all animated beings. Of what benefit, indeed, could life be; what pleafure, what comfort could we enjoy, in the horrors of perpetual darknefs? How could we provide ourselves with food, and the other neceffaries of life? How could we tranfact the least business? How could we correfpond with each other, or be of the leaft reciprocal fervice, without light, and thofe admirable organs of the body, which the Omnipotent Creator has adapted to the perception of this ineftimable benefit?

But it is unneceffary to enumerate the inexpreffible advantages of a blessing, of which the most inattentive of mankind must be fenfible. What is applied to Wifdom, in the book called The Wisdom of Solomon," She is a pure influence flowing from the glory of the Almighty-fhe is the brightnefs of the everlafting Light", has been applied by Milton to Light, with the moft beautiful propriety:

Bright effluence of bright effence increate.

In a word, when we confider the wonderful beauty and pleafures of which light is the effential fource, with how much is ftill involved in mystery, notwithstanding the moft diligent inquiries into its nature and properties, by the moft illuftrious philofophers, well may we exclaim in the beautiful language of Thomson :

How then fhall I attempt to fing of Him
Who, Light Himfelf, in uncreated light
Invefted deep, dwells awfully retir'd
From mortal eye, or angel's purer ken;
Whofe fingle fmile has, from the first of time,
Fill'd, overflowing, all yon lamps of heaven,
That beam for ever through the boundless fky:
But, fhould He hide his face, th' aftonifh'd fun,
And all th' extinguifh'd ftars, would loofening recl
Wide from their spheres, and Chaos come again.

XLV. ON COLOURS.

Colours are but the phantoms of the day,
With that they're born, with that they fade away;
Like beauty's charms, they but amufe the fight,
Dark in themselves, till by reflection bright;
With the fun's aid to rival him they boaft,
But light withdrawn, in their own fhades are loft.

HUGHES.

THE inquiry, in my former paper, into the nature and properties of light, leads me to confider next the admirable relations which the Deity has eftablished between light itself and the furfaces of different bodies, whence proceed the various phenomenons of colours.

Different are the opinions of ancient and modern authors, and of the feveral fects of philofophers, with regard to the nature and origin of the phenomenon Colour. The most popular opinion was long that of Ariftotle, who maintained colour to be a property refiding in the coloured body, and to exist independantly of light. But to this doctrine it was objected, that the neck and feathers of a pigeon or a peacock, change their colours, according to their pofitions. Thus Lucretius:

Pluma columbarum quo pacto in fole videtur, &c.
LIB. II. 800-806.-

The plumes that go around the pigeon's head
Sometimes look brifker with a deeper red;
And then, in different pofitions feen,
Show a gay fky, all intermix'd with green:
And fo in peacocks tails, all fill'd with light,
The colour varies with the change of fite.

CREECH.

It is now univerfally admitted, that colour is a property inherent in light, whereby, according to