498 The Book of the Balance of Wisdom . [October* 
issue forth from the eye to impinge on external things, as up 
to his time had been taught. 
This explanation, moreover, was not based on mere hypo- 
thesis, but was the result of anatomical investigations as 
well as of mathematical discussions. 
Alhazen also explained the astronomical refraction of light, 
its dependence on the variation of the density of the media 
traversed, and its influence in producing the phenomenon of 
twilight. In the discussion of all these problems he evinced 
true scientific greatness. He favoured the theory of the pro- 
gressive development of animal forms, anticipating a doctrine 
but newly obtaining acceptance. Dr. J. W. Draper,* who 
has been our guide in this connection, says of Alhazen : 
“Though more than seven centuries part him from our 
times, the physiologists of this age may accept him as their 
compeer. 
The name al-Khazini signifies “ related to the treasurer,” 
which accords with his statement that the work was com- 
posed for the royal treasury. 
The Book of the Balance of Wisdom begins with a dedica- 
tion to God “ the compassionate, the merciful,” and a pious 
statement of the author’s religious faith. An introduction, 
divided into eight sections, then follows ; in the first section, 
the advantages and uses of the Balance are enumerated in 
this language : 
“These advantages are : — 1, exactness in weighing ; this 
balance shows variations to the extent of a mithkal,+ or of 
a grain, although the entire weight is a thousand mithkals, 
provided the maker has a delicate hand, attends to the 
minute details of the mechanism, and understands it ; 
2, that it distinguishes pure metal from its counterfeit, each 
being recognised by itself without any refining ; 3, that it 
leads to a knowledge of the constituents of a metallic body 
without separation one from another ; 4, that it 
shows the superiority in weight of one of two metals over 
the other in water, when their weight in air is the same, and 
reversely ; 5, that it makes the substance of the 
thing weighed to be known by its weight ; 
6, ; 7, the gain above all others — —that it enables 
* Hist. Int. Devel. Europe, page 360. 
t We connot here undertake to discuss the ancient Arabic system of weights, 
but merely state that while authorities differ, M. Khanikoff, after a careful 
examination and comparison of modern and ancient standards of weights in 
Georgia, Daghistan, and elsewhere, where Arabic customs have suffered little 
change, assigns to the mithkal the value of 4*5 grammes. The mithkal, 
according to Abu-r-Raihan (quoted below) equals 6 daniks ; i danik = q tassujs ; 
i tassuj =4 grains ; and i grain = q barley-corns. 
