Bodies are also divided into transparent and opaque, in reference to their capacity for transmitting light through their substance, though this property depends, not merely on combination of crown and flint glass. He supposed the mean refractive power of flint glass, as compared to crown glass, to be 158 to 153 ; Fratinhofer states it to be 164 to 153. The prismatic dispersion in English flint glass is 1 z times, but in Fraiinhofer's, it is twice as great as in crown glass.
Such are the chief facts concerning light, which bear reference to vision : to the con sideration of the physiology of which we now proceed.
Phenomena of special function with which the retina is endowed being the perception of light, a marvellous range of phenomena is open to the inquirer. It is indeed a wonderful thing to have ascertained beyond doubt, that in perceiving the tint of the scarlet geranium our eyes are affected by undulations recurring four hundred and eighty two millions of millions of times in a second ; ' that before we can appreciate the tint of the yellow blossoms of the gorse or laburnum, five hundred and forty-two millions of mil lions of vibrations must have taken place ; and that to discriminate the colour of the violet, not less than seven hundred and seven millions of millions of movements must have been communicated to the fibrilla; of our re tinm ! Whilst such facts almost transcend the powers of human conception, their esta blishment is a striking triumph of human in tellect. But how great ought to be our admiration of that Omnipotence which has endowed the eye with the gift, not merely of appreciating one colour, but of distinguishing, in all their shades, the inexpressibly compli cated vibrations which mark the hues of a parterre of flowers, and characterise the gor geous plumage of the birds which give anima tion to a tropical forest. The sense of sight, in its ordinary acceptation, may be defined as the recognition by the mind of certain im pressions made upon the retina, and commu nicated through the medium of the optic nerve to the encephalon ; a sound condition of each and all of these parts (which may be considered as the media of communication, so far as one sense is concerned, between the external world and the mind) is indispensable for perfect vision. Light may fall upon the retina, and the images of objects may be there depicted, but should the optic nerve be un sound, or certain portions of the brain be disorganised, no responsive image is called up before the mind ; the eye may gaze upon the noonday sun, but all is dark within.
The natural stimulus of the retina is the luminous rays : the appreciation of light and colour its active condition ; and its state of repose suggests the appearance of darkness : but besides light, any other excitement ap plied to the retina or optic nerve gives rise to the same result,—the production of luminous appearances. Pressure upon the eye-ball, the electric current, or vascular congestion, all excite this special phenomenon. Occasion ally, too, irritation of the brain has the same effect ; and many are the waves and corrus cations, the fiery clouds and flaming spectra, which haunt the amaurotic when certain morbid complications exist. The phantasms of fever, and the illusions of the dying, are to be placed in the same category with the above.
For visual purposes, a certain amount of stimulation of the retina is necessary ; hence it is both difficult and painful to discern ob jects in very faint light : but in this respect the retina is endowed with great powers of accommodation. It is well known that per sons long immured in dungeons profoundly dark to ordinary eyes, have acquired the power of discerning the most minute objects. On the other hand, excessive light is in jurious, for by it vision may be instantane•• ously extinguished, as happens by a stroke of lightning. The accommodation to various amounts of light is, however, gradual, as is proved by passing from a dark room into sun light, or into a brilliantly lighted apartment. In this case the pupil has been widely dilated to admit the greatest possible quantity of light into the eye. For the first second or two after arriving in the light, all is confusion; and there is even pain, from the flood of light which thus breaks in on the retina through the widely dilated pupil, and takes it, as it were, unawares. On the other hand, a per son entering a moderately dark room from full sunlight, sees nothing for a time, because the strongly contracted pupil admits so little light, that no sensation is produced. After a while the pupil dilates, and vision gradually becomes complete. In order that a clear idea may be formed of the mode in which images are depicted upon the retina, and also of the causes of myopia and presbyopia, it is neces sary that the mode of action of convex lenses should be described.