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 eye by the operation called couching, and allow its place to be filled by the aqueous humour.

In this case it is found that convex glasses are necessary to make the sight distinct, which shows that the refracting power of the chrystalline lens is stronger than that of the humour which is here substituted for it.

143. It may be observed, that all persons requiring either concave or convex lenses to assist their sight, should always chuse those of the least refracting power that will answer the purpose, as the use of them tends to increase the defect they are intended to remedy, namely, a defect in the power of accommodating the eye to the perception of objects at different distances from it.

144. The fact of images of external objects being produced by the eye, and serving as the medium of vision, has led to a great deal of discussion about the manner in which we take cognizance of external objects by the help of the senses. Sir I. Newton published in his Optics the following query, among others: "Do not the rays of light in falling upon the bottom of the eye, excite vibrations in the tunica retina? which vibrations, being propagated along the solid fibres of the nerves into the brain, cause the sense of seeing." And again, "When a man in the dark presses either corner of his eye with his finger, and turns his eye away from his finger, he will see a circle of colours like those in a peacock’s tail. Do not these colours arise from such motions excited in the bottom of the eye by the pressure of the finger as at other times are excited there by light for causing vision? And when a man by a stroke upon his eye sees a flash of light, are not the like motions excited in the retina by the stroke?" We may give the above as a specimen of some of the more sane speculations on this subject, with respect to which, as to many others connected with the reciprocal actions of mind and matter, the only knowledge at which the most profound philosophers have arrived, is, that like Socrates, they know nothing.