power, grandly wrote in the beautiful mosaics on their palace walls, an ever-recurring warning to him who owes dominion to the sword, "There is no conqueror but God."

The value of a philosophical or political system may be determined by its fruits. On this principle I examined in Vol. I., Chapter XII., the Italian system, estimating its religious merit from the biographies of the popes, Examination which afford the proper criterion. In like manner, of Mohamthe intellectual state of the Mohammedan nations medan at successive epochs may be ascertained from what is its proper criterion, the contemporaneous scientific manifestation.

science.

At the time when the Moorish influences in Spain began to exert a pressure on the Italian system, there were several scientific writers, fragments of whose works have descended to us. As an architect may judge of the skill of the ancient Egyptians in his art from a study of the Pyramids, so from these relics of Saracenic learning we may demonstrate the intellectual state of the Mohammedan people, though much of their work has been lost and more has been purposely destroyed.

Among such writers is Alhazen; his date was about A.D. 1100. It appears that he resided both in Spain and Egypt, but the details of his biography Review of the are very confused. Through his optical works, works of which have been translated into Latin, he is Alhazen. best known to Europe. He was the first to correct the Greek misconception as to the nature of vision, showing that the rays of light come from external objects to the eye, and do not issue forth from the eye, and He corrects impinge on external things, as, up to his time, the theory of had been supposed. His explanation does not vision. depend upon mere hypothesis or supposition, but is plainly based upon anatomical investigation as well as on geometrical discussion. He determines that the retina Determines is the seat of vision, and that impressions made the function 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,

of the retina.

Traces the

course of a ray of light through the

air.

nor, indeed, know anything about these facts, unless he had been engaged in the forbidden practice of dissection. Explains sin- With felicity he explains that we see single gle vision. 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 trustworthy guide, and that there are illusions arising from the course which the rays of light may take when they suffer refraction or reflexion. 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 Astronomical term, nearer to the zenith than they actually refraction. 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 vapours 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 tal sun and 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

The horizon

moon.

reflecting action of the air, he deduces that beautiful explanation of the nature of twilight-the light Explains the that we perceive before the rising and after the twilight. setting of the sun-which we accept at the present time as true. With extraordinary acuteness, he ap- Determines plies the principles with which he is dealing to the atmosthe determination of the height of the atmos- phere. phere, deciding that its limit is nearly 58 miles.

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, be the production of Alhazen, of which 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 The weight of set forth the connexion between the weight of the air. the atmosphere and its increasing density. The weight of the atmosphere was therefore understood before Torricelli. This author shows that a body will weigh differently in a rare and in a dense atmosphere; that its loss of Principles of weight will be greater in proportion as the air is hydrostatics. 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 Theory of the investigation of balances and steelyards, show- balance. ing 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

Gravity; capillary attraction; the hy

drometer.

Tables of

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 hydrometerthe instrument which, in a letter to his fair but pagan friend Hypatia, the good Bishop of Ptolemais, Synesius, six hundred years previously, 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 specific gra- given by Alhazen, approach very closely to our vities. 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 ask the same for Alhazen himself, since 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 development of animal forms. He upheld the affirmation of those who The theory of said that man, in his progress, passes through a development definite succession of states; not, however, "that of organisms. he was once a bull, and was then changed to an ass, and afterwards into a horse, and after that into an ape, and finally became a man.' This, he says, is only a misrepresentation by "common people" of what is really meant. The "common people" who withstood Alhazen have representatives among us, themselves the only example in the Fauna of the world of that non-development which they so loudly affirm. At the best they are only passing through some of the earlier forms of that series of transmutations to which the devout Mohammedan in the above quotation alludes.

The Arabians, with all this physical knowledge, do not appear to have been in possession of the thermometer, though they knew the great importance of temperature measures,

Junis.

employing the areometer for that purpose. They had detected the variation in density of liquids by heat, but not the variation in volume. In their measures of time they were more successful; they had several kinds of clepsydras. A balance clepsydra is described in the work from which I am quoting. But it was their great astronomer, Ebn Junis, who accomplished the most valuable of all chronometric improvements. He first The penduapplied the pendulum to the measure of time. lum clock. Laplace, in the fifth note to his Systeme du Monde, avails himself of the observations of this philosopher, with those of Albategnius and other Arabians, as incontestable proof of the diminution of the eccentricity of the earth's Astronomical orbit. He states, moreover, that the observation works of Ebn of Ebn Junis of the obliquity of the ecliptic, properly corrected for parallax and refraction, gives for the year A.D. 1000 a result closely approaching to the theoretical. He also mentions another observation of Ebn Junis, October 31, A.D. 1007, as of much importance in reference to the great inequalities of Jupiter and Saturn. I have already remarked that, in the writings of this great Arabian, the Arabic numerals and our common The Arabic arithmetical processes are currently used. From numerals. Africa and Spain they passed into Italy, finding ready acceptance among commercial men, who recognised at once their value, and, as William of Malmesbury says, being a wonderful relief to the "sweating calculators;" an epithet of which the correctness will soon appear to any one who will try to do a common multiplication or division problem by the aid of the old Roman numerals. It is said that Gerbert-Pope Sylvester-was the first to introduce a knowledge of them into Europe; he had learned them at the Mohammedan university of Cordova. It is in allusion to the cipher, which follows the 9, but which, added to any of the other digits, increases by tenfold its power, that, in a letter to his patron, the Emperor Otho III., with humility he playfully but truly says, "I am like the last of all the numbers."

The overthrow of the Roman by the Arabic numerals foreshadowed the result of a far more important-a political-contest between those rival names. But, before