Page:Proceedings of the Royal Society of London Vol 1.djvu/55

Rh thimble on one of the fingers. This deprived her of sight for ten days, after which, having had a severe hysterical paroxysm, her vision re- turned, but convulsive and hysterical sensations still remained, from which she was not relieved till six weeks after. This case, which the author conceives to be a proof that the electric fluid is capable of sus- pending the functions of the optic nerve without altogether destroy ing them, suggested the idea of some further experiments, with a view to ascertain whether electricity could be so applied, by artificial means, as to destroy the power of contraction possessed by nerves. The effects seemed to prove the negative; but it is owned these ex- periments were made under circumstances which did not inspire much confidence.

It appears, in general, that these experiments, and the observations deduced from them, materially illustrate an action or power inherent in the nervous chords, capable of producing the symptoms which oc- curred in the cases here related; and that the once favourite hypo- thesis of a nervous fluid, does not give a satisfactory solution of those nervous agitations, which only proceed for some way in the course of a nerve, and are there arrested without being allowed to proceed to the brain. The disorder known by the name Tic Douloureux, is given as an example of the manner in which spasmodic tremors are propagated along the nerves. And a case of a locked jaw occasioned by an injury to the thumb, is lastly mentioned, which corroborates all that has been said concerning the first case mentioned in this раper.

The copious contents of this lecture relate chiefly to the power pos- sessed by the eye to accommodate itself to the perception of objects at different distances, concerning which a variety of opinions have been entertained.

After some general considerations on the sense of vision, from which it appears that though the extent of the field of perfect vision for each position of the eye be not very great, yet there is reason to believe that its refractive powers are calculated to take in a moderately distinct view of a whole hemisphere: the author, aware how essential it is in an inquiry of so delicate a nature to proceed upon solid, and as far as possible incontrovertible grounds, delivers a set of dioptrical propositions (eight in number), each accompanied by some scholia and corollaries, from which he means to deduce the principal infer- ences brought forward in the sequel of the lecture.

The nature of these can only be here intimated by their different enunciations, some of which may appear elementary, yet lead to results of a less obvious nature. They are as follows;-1. In all refractions the ratio of the sine of the angle of incidence to the sine of the angle of refraction, is constant. 2. If between two refracting mediums a third medium, terminated by parallel surfaces, be inter-