74

HISTORICAL VIEW OF THE

which the subject of refraction is accurately enquiredinto. This treatise, though known in the middle ages,and quoted by Roger Bacon , had disappeared, and wassupposed to be entirely lost, till, in very recent times, acopy (professedly a translation from the Arabic ) wasfound in the king’s library at Paris: another copy alsoexists in the Savilian library at Oxford.

Ptolemy , actuated, doubtless, by the necessity he felt,as an astronomer, for a more accurate knowledge ofatmospheric refraction, as affecting the places of theheavenly bodies, examined with great care and precisionthe angles of refraction corresponding to all angles ofincidence from 0 to 80 degrees, when a ray enters aa medium of water, or of glass, out of air. The theorywhich he hence gave of astronomical refractions was evenmore correct than that adopted by some of the moderns.In an optical point of view, his measurements accordvery exactly with the modern law of the sines: though hedid not advance to any such generalisation from them.

Some modern writers (probably from incorrect ver-sions of Ptolemy 's work), ascribe to him the explanationof the fact, that the sun and moon on the horizon ap-pear larger than in the zenith, as depending on the cir-cumstance that, in the former case, our judgment isinfluenced by comparison with terrestrial objects. Ithas been ascertained by De Lambre, that in the originalhe does not give any thing like this explanation (whichis, doubtless, the correct one), but only a very vaguetheory. This explanation is, therefore, probably dueto his Arabic translator ; and most likely derived fromAlhazen .

Ptolemy distinguishes what has since been called thevirtual focus, in the reflection from convex specula, orthe point where the reflected rays, if produced, wouldmeet. And, among other interesting glimpses of truthsnot fully discovered till many ages afterwards, he noticesthe fact, that colours are confounded together by therapidity of motion; and gives the instance of a wheelpainted with different colours, and turned quickly round.