PROGRESS AMONG THE ARABS. 35 



gnomon 180 feet high, determined the obliquity of the 

 ecliptic to be 23 30' 20" (?) ; the precession of the equinoxes 

 at 1 in seventy years (which gives 25,200 YEARS for the 

 entire cycle a marvellous approximation) ; drew up tables 

 scarcely inferior in accuracy to those of Tycho Brahe. 



The preceding table enables us to see by how much the 

 Arabs extended Greek astronomy and mathematics. Their 

 originality and genius, however, are far more striking in 

 physics. The discoveries made by Alhazen alone, if made 

 amongst us at the present day, would make a profound 

 impression, and would be considered astounding additions to 

 modern science. It is difficult to understand how this great 

 man arrived at such results ; he was the scientific man par 

 excellence of the Arabian race, just as Hipparchus was 

 of the Greek, Galileo of the Italian, and Newton of the 

 English. 



noo. Alhazen was at once a geometrician, a PHYSICIST, 

 and a physiologist. He was best known in Europe by his 

 Optics his works having been put into Latin comparatively 

 early. 



a. By anatomical and geometrical investigations he 

 corrected the Greek mistake regarding the nature of vision, 

 showing that light-rays come from objects to the eye, and do 

 not issue forth from the organ ; 



b. Determined the function of the retina, showing that 

 the impressions upon it are conveyed to the brain through 

 the optic nerve a discovery which implies dissection ; 



c. Explained SINGLE VISION, by the fact that im- 

 pressions are made on symmetrical portions of the two 

 retinas. 



d. He discovered REFRACTION, and showed the untrust- 

 worthiness of our sense of sight, and rightly ascribed illusion 

 to refraction and reflection a discovery of high order, by 

 which he was able to explain numerous phenomena, and 

 which ranks him among the greatest physicists of all times 

 and countries ; 



e. Discovered that the DENSITY OF THE ATMOSPHERE 

 decreases with increase of height ; 



/. Explained the curvilinear course of a ray of light 



D 2 



