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There are several other circumstances to be taken into account as giving some of its peculiar character to thunder. The original sounds are probably reverberated, sometimes from the clouds and sometimes from the earth. But a more influential circumstance is, that the reports from different points in the track of the lightning reach the ear, not at once, but in succession.

Sound, as we all know, travels much more slowly than light, and it travels, also, much more slowly than the electric fluid. If we suppose a flash of lightning to pursue an uninterrupted track receding from the place where we stand, as its passage would be almost instantaneous, the report must be equally instantaneous along the whole line; but the sound from the different points of the line not falling on our ears at the same moment, we should hear a prolonged peal instead of an instantaneous clap.

Such appears to me to be the true theory of the production of thunder considered as a sound. Different views will probably be taken of the mode in which the vacuum, that constitutes so important a part of the process, is formed; but, whatever opinion may be entertained on this point, it will still be a correct account of the causes of thunder to say, that the sound is owing jointly to the im. pact of the electric fluid on the air, and to the rush of the air into the vacuum which the passage of the electric fluid occasions; in addition to which

there is of course, on many occasions, the sound arising from the impact of the lightning on the solid substances of the earth.

In closing my discourse, it may be well, by way of apology for one of its features, to mention, that its brevity is owing to an arrangement between another member of the Society and myself, by which, instead of either of us occupying the whole evening with one long dissertation, each undertook to furnish a short essay. Thus restricted, I found it expedient to confine myself to the elucidation of a single question amongst the many interesting topics presented by the science to which it belongs.

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DISCOURSE VIII.

ON THE PARADOXES OF VISION.

AMONGST the difficulties connected with the physiology of vision, there are several of an interesting character; and, amongst these, there are two especially which have for a long period excited discussion and controversy, without having been hitherto settled or solved to general satisfaction.

These, in the brief but inexact language usually employed, are described as erect vision with inverted images, and single vision with two eyes.

It is well known to all who have the slightest knowledge of Optics, that the images or pictures painted on the retina are inverted, and that, when we see an object most perfectly, there is a picture of it in each eye, although the object is seen single.

The difficulties presented by these facts, which had formerly engaged the curiosity of philosophers in no ordinary degree, seem to have been neglected for an interval of some years, till attention was not long ago recalled to them by Mr. Wheatstone, Mr. Whewell, Sir D. Brewster, and others.

Mr. Wheatstone, in a very interesting and original paper in the Philosophical Transactions on

Binocular Vision*, has brought forward a number of striking facts bearing on the second question, which have placed it in quite a new light.

To these, and to some inferences flowing from them, which appear to me to have been overlooked, I shall hereafter more particularly call your attention.

Mr. Whewell, in his able work on the "Philosophy of the Inductive Sciences," has discussed both of the difficult questions before us. He terms them the Paradoxes of Vision, and has treated them and some kindred topics, if not with all the exactness desirable, at least with a free and powerful pen. Like every other writer of any celebrity, he soon found antagonists to contest some of his positions. His treatise had not been long published before the "Edinburgh Review" took him roundly to task, and went pretty largely into the discussion of these and collateral inquiries. I have no information at all as to the authorship of the article referred to, which appeared in the number for January last†; but, from internal evidence, there can be little hesitation in attributing it to one of the most distinguished men of science in Scotland.

The two paradoxes of vision (to adopt Mr. Whewell's convenient phraseology) having been thus recently brought before the great philosophic world

* Philosophical Transactions, 1838, p. 371.
† No. 152. January 1842.

by writers of so much ability, it occurred to me that a paper on the subject might possibly be interesting to our smaller sphere, especially as I conceive some of their explanations and arguments to be unsuccessful or erroneous; and that there is room for a completer solution of some difficulties, and a more accurate statement of others, than any which has yet appeared.

Although I cannot hope to make a discussion of so abstruse a nature intelligible to persons altogether unacquainted with physiological inquiries and metaphysical speculation, I flatter myself I shall be able to place the subject in a clearer light than that in which it has been hitherto exhibited of my auditors.

to many

Let us, in the first place, consider the paradox of erect vision with inverted images.

The celebrated philosopher Kepler, although anticipated in some respects by Baptista Porta, has the credit of having first made the discovery, that inverted pictures of visible objects are painted upon the retina; and he showed from the principles of Optics how these pictures are necessarily formed in that position. The rays coming from any one point of an object, and falling divergently on the eye, are refracted by the cornea and crystalline, so as to unite again on some point of the retina, painting there the colour of that point of the object whence they come. As the rays from different points of the object cross each other before they

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