That it excites to increased action the arteries of the tongue in the experiment, in which a sense of warmth is produced along its surface by the application of the metals to the lips, seems to be almost demonstrated; for it would be difficult to point out the presence of another cause competent to occasion the evolution of the heat, in this case, besides the increased action of the arteries: and that this cause is competent to the effect we know from numberless experiments, too familiar to need being particularized here. Whether the metals, however, do or do not affect the action of the blood vessels, is a question which admits of solution by experiment. The following, I confess, was not quite satisfactory, and I have not yet found leisure and opportunity to repeat it with all the attention it requires. I inspected the foot of a living frog with a microscope of very high powers. In fixing the foot so as to keep the web expanded, a considerable degree of inflammation was excited, notwithstanding every precaution to avoid it. The current of blood was seen distinctly in several vessels, now flowing rapidly, now slowly, and now in a direction contrary to that in which it was first observed, but with equal rapidity. A thin plate of zinc was introduced between the fleshy part of the foot and its supporter, and a silver probe was used as an excitor. To me, the circulation appeared very decidedly to be quickened several times when the metals were made to touch each other: but the gentlemen who assisted me could observe no change. To prevent the contractions in the muscles of the leg from producing any fallacy, the crural artery should be laid bare, and insulated from surrounding parts, by passing a thin plate of glass, or sealing wax, between it and them. That the flash is the effect of such an increased action of the vessels, composing the choroid coat, might be somewhat more difficult to prove. It is however known to every one, that a blow, and that pressure upon the eye, are capable, as I have before observed, of producing a similar effect. And the following case, which Bonetus quotes from Hermannus Cummius, if it may be credited, affords an almost positive proof, that vision depends upon the stimulus given to the retina by the activity of blood vessels in some part of the eye. ‘Quando theologus, plaga dolorifica, a rupta instrumenti musici chorda accepta, nocte subsequenti jam adulta, e somno evigilans, cuncta clare, ac si de die esset, vidit, adeo, ut minimos picturarum et tapetum tractus observare, characteresque ex libro legere posset. Oculo vero læso clauso, tenebras densissimas adesse ille percepit, eodemque iterum aperto, conclave illustratum visum est, lucem tamen candelæ allatae solisque splendorem de die, ægre tulit oculus affectus, quod per aliquot dies duravit, tandemque sensim remisit.’ Haller speaks of such cases as by no means uncommon, and quotes the names of several authors, who have related similar ones.