little water, or a wet finger, or, still better, any black substance wetted, to the posterior face, at the spot where the internal reflection takes place, and the second image will immediately lose great part of its brightness. If olive-oil be applied instead of water, the defalcation of light will be much greater; and if the substance applied be pitch, softened by heat, so as to make it adhere, the second image will be totally obliterated. On the other hand, if we apply substances of a higher refractive power than glass, the second image again appears. Thus, with oil of cassia it is considerably bright; with sulphur, it cannot be distinguished from that reflected at the first surface; and if we apply mercury, or amalgam (as in a silvered looking-glass), the reflection at the common surface of the glass and metal is much more vivid than that reflected from the glass alone."

I close these remarks and experiments on light by another interesting passage on the velocity of light, by the same celebrated philosopher and astronomer.

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Light requires time for its propagation. Two spectators at different distances from a luminous object suddenly disclosed, will not begin to see it at the same mathematical instant

of time. The nearer will see it sooner than the more remote; in the same way as two persons at unequal distances from a gun hear the report at different moments. In like manner, if a luminous object be suddenly extinguished, a spectator will continue to see it for a certain time afterwards, as if it still continued luminous, and this time will be greater the farther he is from it. The interval in question is, however, so excessively small in such distances as occur on the earth's surface, as to be absolutely insensible; but in the immense expanse of the celestial regions the case is different. The eclipses and emersions of Jupiter's satellites become visible much sooner (nearly a quarter of an hour) when the earth is at its least distance from Jupiter than when at its greatest. Light, then, takes time to travel over space, It has a finite though immense velocity, viz. 192,500 miles per second; and this important conclusion, deduced, by calculation, from the phenomenon just mentioned, and which, if it stood unsupported, might startle us with its vastness, and incline us to look out for some other mode of explanation, receives full confirmation from another astronomical phenomenon, viz. the aberration of light."

CHAPTER VI.

ON THE PRESERVATION OF SIGHT; AND ON THE USE, ABUSE, AND CHOICE OF SPECTACLES, &c.

My design in this chapter is to give in plain and easily understood language, some advice on the care necessary to be taken of the eyes, on the means of restoring their healthy action when impaired by over-exertion, and to offer some remarks on the use, abuse, and choice of spectacles.

In proportion to the expansion of the pupil of the eye, is the sensibility of the organ: the mean diameter of the pupil, though varying from one to two tenths of an inch in proportion to the brightness of objects, is reckoned to be commonly about one tenth of an inch.

When the light is too strong, or the object too bright, the pupil contracts, that it may intercept the excess of light, by which the eye

would otherwise be distressed on the contrary, when the light is faint, the pupil expands, in order that a larger portion of it may be admitted by the eye, and thus a more powerful impression be made upon it.

Sudden changes from comparative darkness to strong light, and vice versa, are highly improper hence the eyes should be carefully guarded from the full effect of the morning sun on first awaking in summer; and the custom of breakfasting in the lightest room in the house, as is generally practised, is certainly weakening to the eyes, which ought to be accustomed by gentle transitions from one degree of light to another, till they can bear the effulgence of the sun's meridian splendour.

Rubbing the eyes on waking is a destructive habit which many people have contracted; for though healthy persons, whose sight is moderately used through the day, may not be sensible of receiving any injury from this custom; yet those whose occupations demand close application of their visual organs for any continued space of time, will soon be convinced by painful experience of the truth of this remark. Besides the daily injury thus done to the eyes, it sometimes also happens that hairs and other

foreign matters are forced into them by their being violently rubbed, which may occasion inflammation, and are frequently very troublesome to dislodge. The inflamed and weak eyes of many persons are likewise in a great measure to be attributed primarily to this most imprudent habit. Should, however, the eyelids be so fixed that a difficulty in opening them is felt, let them be moistened with a little warm milk. and water for a few minutes, which, in all cases where the organ is healthy, will be found to answer the purpose in a manner such as they can have no idea of who have never tried this simple remedy.

The use of shades and bandages, on every trifling affection of the eye, is an evil that cannot be too strongly reprobated; for the action of light and air being thus excluded, and the organ rigidly compressed, ophthalmia, and even total blindness, is not infrequently the consequence of what, being perhaps merely a slight flow of humour, or a little extravasated blood, would have subsided in a few days, if judiciously treated, or even if left to itself.

Bathing the eyes occasionally during the day as well as on rising, is of much importance to their preservation: where the organ is