EMILY.

And supposing that one of the rays were a whole wave in advance of the other, would light or darkness be the result of their union?

MRS. B.

It would be light; because the waves, though unequal in number, would coincide. Whatever number of whole waves be added to one of the rays, light is produced by its junction with the other ray; but the addition of any fractional part of a wave produces an interference which extinguishes the light of both rays.

This is one of the strongest arguments in favour of the undulating theory of light, in opposition to that of the corpuscular theory; for it is contrary to all our ideas of matter to suppose that two particles coming in contact should annihilate one another, whilst it is very natural to suppose that the contact of two particles may destroy each other's motion.

EMILY.

Oh, yes! I have seen two billiard balls, when struck with equal force from different ends of the table, destroy each other's motion at the point of contact, but they could not destroy each other's substance.

MRS. B.

Neither could the smallest particles of matter do so; if, therefore, light were corporeal, it could

not be extinguished by two particles coming in contact under any circumstances.

There is another property of light, which I cannot pass over in silence, though it will not admit of a familiar explanation any more than the interference of the rays or the polarization of light.

If a ray of light pass through a pin-hole into a dark room, and be received on a white skreen at the distance of a little more than six feet, a luminous spot will be described on the skreen, larger than the pin-hole through which the light passes, and this will be surrounded by a series of coloured rings separated from each other by dark intervals.

How very curious!

EMILY.

MRS. B.

Similar phenomena are produced when rays pass through a very narrow slit, excepting that, instead of coloured rings, coloured stripes are formed, separated by dark ones.

This is explained on the principle of the interference of the rays: the brilliant coloured rings being the result of the junction of rays whose waves coincide, and the dark intervals being produced by the mutual distinction of rays whose waves do not coincide.

CAROLINE.

There seems to me to be some analogy between the undulations of light and those of sound.

MRS. B.

The analogy is very remarkable, and the deeper the subject is studied, the more perfect it appears. The principle of the interference of rays, which is now fully established, has led to the explanation of many of the properties of light which were previously not understood. But I cannot attempt to follow up such investigations: it is sufficient for the present that I should point out the facts, in case that, at any future time, you should be induced to prosecute the study more deeply.

443

CONVERSATION XVIII.

OPTICS.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.

THE RETINA. — REFRACTION OF THE HUMOURS OF OF THE USE OF SPECTACLES. -OF

THE EYE.

I SHALL this morning give you some account of the structure of the eye; you have hitherto considered it only as a simple camera obscura, in which the representation of objects is made on the retina; but I must now tell you, that this camera obscura is furnished with a variety of substances, all of which contribute to the perfection of vision, and is enclosed in a double covering to guard it from injury.

The body of the eye is of a spherical form (fig. 1. Plate XXI.); it has two membraneous coverings: the external one, a a a, is called the sclerotica: this has a projection in that part of the eye which is exposed to view, b b, which is called the cornea, because, when dried, it has nearly the consistence of very fine horn, and is sufficiently transparent for the light to obtain free passage through it.

The second membrane, which lines the cornea, and envelopes the eye, is called the choroid, ccc; this has an opening in front just beneath the cornea, which forms the pupil, dd, through which the rays of light pass into the eye. The pupil is surrounded by a coloured border of fibres, called the iris, ee, which, by its motion, always preserves the pupil of a circular form, whether it be expanded in the dark, or contracted by a strong light. This you will understand better by examining fig. 2.

EMILY.

I did not know that the pupil was susceptible of varying its dimensions.

MRS. B.

The construction of the eye is so admirable, that it is capable of adapting itself, more or less, to the circumstances in which it is placed. In a faint light the pupil dilates so as to receive an additional