REPORT ON OPTICS. 
309 
optics. It was not till the second century of the Christian sera, 
that Claudius Ptolemy, the celebrated astronomer of Alexandria, 
laid the foundation of this branch of the science, by an accurate 
examination of the phenomena of refraction. This distinguished 
individual, to whom Astronomy owes such deep obligations, mea¬ 
sured with singular exactness the angles of refraction in water 
and glass for various incidences from 0° to 80°, and he deter¬ 
mined the same angles when the light passed from the one medi¬ 
um into the other. These inquiries were no doubt undertaken in 
reference to the refractions of the atmosphere, in which, as an 
astronomer, he felt the greatest interest; and such was the suc¬ 
cess with which he applied them to this important object, that he 
gave a theory of astronomical refractions more complete than that 
of any other astronomer before the time of Cassini. While Tycho 
in the fifteenth century believed that the refraction of the atmo¬ 
sphere terminated at an altitude of 45°, and while others placed 
the zero of refraction at the pole of the ecliptic, Ptolemy had 
shown 1200 years earlier, that the refraction increased gradu¬ 
ally from the zenith to the horizon, and that all stars were ele¬ 
vated by it above their true place : nay, he uses the very same 
diagram upon which Cassini has founded his theory, and he 
employs almost the same reasoning as the French astronomer in 
order to determine the quantity of refraction. 
Although the “Optics” of Ptolemy, in which these discove¬ 
ries are recorded, is known to have existed in the time of Roger 
Bacon, yet the work appears to have been lost sight of by his 
successors, till two copies of it were lately found, one in the 
Savilian Library at Oxford, and the other in the Imperial Li¬ 
brary at Paris. 
The “ Optics ” of Alhazen and of his disciple Vitello, though 
written a thousand years after that of Ptolemy, contain but very 
trifling additions to the science. Vitello, indeed, obtained 
more accurate measures of the deviation of the refracted ray 
in glass and water; but though his numbers were sufficiently 
exact for the purpose, he did not discover the constant relation 
which exists between the sines of incidence and refraction. 
The discovery of this important law was reserved for Wille- 
brord Snellius, professor of mathematics at Leyden, a young 
man of the finest genius, who died at the age of 35, before he 
was able to give his own account of it to the world. The at¬ 
tempt which was made by Descartes to appropriate to himself 
this discovery is unparalleled in the history of Science ; and 
the means which have been taken in later times to deprive the 
accomplished Snellius of his legitimate and single claim, present 
to us one of the most striking examples of national partiality. 
