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 strong mental emotions. It may also be performed voluntarily; but then the force is much inferior to that of automatic sneezing. The same may be observed of hiccough, coughing, yawning, stretching, &c. and is very agreeable to the derivative nature of these motions, when voluntary, i.e. when performed by motory vibratiuncles. The action of sneezing is differently modelled by voluntary and semi-voluntary powers in different persons.

Section IV
THE SENSE OF SIGHT.



the retina is an expansion of the optic nerve, we may conclude, from the analogy of the other senses, that it is the immediate organ of sight. Nor is the want of sensibility in the button of the optic nerve a sufficient objection to this; as the minute structure and disposition of the parts of this button are not known.

We may also reason thus to the same purpose. It may be expected, that the immediate organ of sight should be either black or white, that so it may bear a relation of indifference to all the colours. But if we admit the doctrine of vibrations, black, by absorbing all kinds of rays, would make a confusion of vibrations, whereas white, by reflecting all, might retain the impressed vibrations distinctly. The retina is therefore peculiarly fitted for the immediate organ of sight, and the choroides the contrary. We may add farther, that the retina, by reflecting rays copiously, prevents their arrival at the choroides.

For the accurate distinction of the several visible points of objects, it is necessary that these be placed within the limits of distinct vision; and also, that the coats and humours of the eye be so circumstanced, as to bring the several pencils of rays, which proceed from each visible point accurately or nearly, to a corresponding point upon the retina. This is distinct vision. But colours alone may be distinguished from each other without any exact conformation of the eye. Thus vision may be reckoned of two kinds, as feeling, taste, and smell, have been.