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REVIEW OF ALHAZEN. 359.
plainly based upon anatomical investigation as well as on geometrical discussion. He determines that the retina is the seat of vision, and that impressions made by light upon it are conveyed along the optic nerve to the brain. Though it might not be convenient, at the time when Alhazen lived, to make such an acknowledgment, no one could come to these conclusions, nor, indeed, know any thing about these facts, unless he had been engaged in the forbidden practice of dissection. With felicity he explains that we see single when we use both eyes, because of the formation of the visual images on symmetrical portions of the two retinas. To the modern physiologist the mere mention of such things is as significant as the occurrence of an arch in the interior of the pyramid is to the architect. But Alhazen shows that our sense of sight is by no means a reliable guide, and that there are illusions arising from the course which the rays of light may take when they suffer refraction or reflection. It is in the discussion of one of these physical problems that his scientific greatness truly shines forth. He is perfectly aware that the atmosphere decreases in density with increase of height; and from that consideration he shows that a ray of light, entering it obliquely, follows a curvilinear path which is concave toward the earth; and that, since the mind refers the position of an object to the direction in which the ray of light from it enters the eye, the result must be an illusion as respects the starry bodies; they appear to us, to use the Arabic term, nearer to the zenith than they actually are, and not in their true place. We see them in the direction of the tangent to the curve of refraction as it reaches the eye. Hence also he shows that we actually see the stars, and the sun, and the moon before they have risen and after they have set a wonderful illusion. He shows that in its passage through the air the curvature of a ray increases with the increasing density, and that its path does not depend on vapors that chance to be present, but on the variation of density in the medium. To this refraction he truly refers the shortening, in their vertical diameter, of the horizontal sun and moon; to its variations he imputes the twinkling of the fixed stars. The apparent increase of size of the former bodies when they are in the horizon he refers to a mental deception, arising from the presence of intervening terrestrial objects. He shows that the effect of refraction is to shorten the duration of night and darkness by prolonging the visibility of the sun, and considering the reflecting action of the air, he deduces that beautiful explanation of the nature of twilight-the light that we perceive before the rising and after the setting of the sun-which we accept at the present time as true. With extraordinary acuteness, he applies the principles with which he is dealing to the determination of the height of the atmosphere, deciding that its limit is nearly 58.5 miles.
360 REVIEW OF ALHAZEN.
All this is very grand. Shall we compare it with the contemporaneous monk miracles and monkish philosophy of Europe? It would make a profound impression if communicated for the first time to a scientific society in our own age. Nor perhaps does his merit end here. If the Book of the Balance of Wisdom, for a translation of which we are indebted to M. Khanikoff, the Russian consul-general at Tabriz, is the production of Alhazen, as there seems to be internal proof, it offers us evidence of a singular clearness in mechanical conception for which we should scarcely have been prepared, and, if it be not his, at all events it indisputably shows the scientific acquirements of his age. In that book is plainly set forth the connection between the weight of the atmosphere and its increasing density. The weight of the atmosphere was therefore understood before Tdrricelli. He shows that a body will weigh differently in a rare and in a dense atmosphere; that its loss of weight will be greater in proportion as the air is more dense. He considers the force with which plunged bodies will rise through heavier media in which they are immersed, and discusses the submergence of floating bodies, as ships upon the sea. He understands the doctrine of the centre of gravity. He applies it to the investigation of balances and steelyards, showing the relations between the centre of gravity and the centre of suspension-when those instruments will set and when they will vibrate. He recognizes gravity as a force; asserts that it diminishes with the distance; but falls into the mistake that the diminution is as the distance, and not as its square. He considers gravity as terrestrial, and fails to perceive that it is universal-that was reserved for Newton. He knows correctly the relation between the velocities, spaces, and times of falling bodies, and has very distinct ideas of capillary attraction. He improves the construction of that old Alexandrian invention, the hydrometer-the instrument which, in a letter to his fair but pagan friend Hypatia, the good Bishop of Ptolemais, Synesius, six hundred years before, requests her to have made for him in Alexandria, as he wishes to try the wines he is using, his health being a little delicate. The determinations of the densities of bodies, as given by Alhazen, approach very closely to our own; in the case of mercury they are even more exact than some of those of the last century. I join, as, doubtless, all natural philosophers will do, in the pious prayer of Alhazen, that, in the day of judgment, the All-Merciful will take pity on the soul of Abur-Raihan, because he was the first of the race of men to construct a table of specific gravities; and I will add Alhazen’s name thereto, for he was the first to trace the curvilinear path of a ray of light through the air. Though more than seven centuries part him from our times, the physiologists of this age may accept him as their compeer, since he received and defended the doctrine, now forcing its way, of the progressive de.

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