PHYSICAL AND MATHEMATICAL SCIENCES. 6?

Seneca; but the dependence of this on the lenticularform of the glass seems to have been wholly unknown;and this case was confounded together with others, in away evincing a total absence of all conception of thecause. Seneca mentions the magnifying power of around glass vessel of water, associated with a statementpurely imaginary, that the stars are magnified by beingseen through a cloud; and ascribes the cause to somepower of humidity. Speculations of the like nature arealso given on the rainbow. Perhaps one of the mostcurious and interesting subjects adverted to by thiswriter is the phenomenon of the prismatic colours. Hedistinctly describes an angular glass rod, evidently of aprismatic form, which, he says, produces colours likethose of the rainbow. This he considers to be occa-sioned from the circumstance that the image is irregu-larly formed; (quia enormiter facta est;) and adds,that such a glass, properly made, will give as manyimages of the sun as it has angles: which is nothingmore than the multiplication of an object, by a glasscut with a number of faces.* He appears to have re-marked, also, the atmospherical refraction, and theelasticity of the air. t

Aulus Gellius (about A. D. 130), among the varietyof subjects he has recorded, has preserved some curiousparticulars relative to practical optics, as known in thatage : in particular, when speaking of the properties ofreflectors, he has mentioned one relating to concavemirrors, which, for a long time, completely baffled theskill of the most scientific commentators to interpret,until it received, very recently, a complete elucidationfrom Mr. Horner.j; This property, so obscurely ex-pressed (as, indeed, are several others of those herediscussed), is extensively available in optical deceptions,and, doubtless, afforded the secret of many a miraculousapparition in an ignorant age. The particular property

» QuffiSt Nat. i. 6. t Lib. v. 8. and vi. 16.

X A forgotten Fact in Optics. Bath, 1832.

F 2