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LXXIII. On Light. By Atss-dk^wUrEjM.D. Professor of the 

 Andersonian Institution, Glasgow*. 



J-iiGHT. The agent of vision. — Some philosophers regard light 

 as consisting of particles of inconceivable minuteness, emitted in 

 snccession bv luminous bodies^ which move in straight lines, at 

 the rate of 200,000 miles per second. 



Others conceive that it consists in certain undulations com- 

 municated by luminous bodies, to an ethereal fluid which fills ail 

 space. This fluid is composed of the most subtile matter, is 

 highly elastic, and the undulations are propagated through it with 

 great velocity, in spherical superficies proceeding from a centre. 

 This view derives great plausibility from its happy application by 

 Huygens, to explain a very difficult class of optical phaenomena, 

 the double refraction of calcareous spar and other bodies. 



The commo7i refraction is explained by Huygens on the sup- 

 position, that the undulations in the luminous fluid are propa- 

 gated in the form of spherical waves. The double refraction 

 is explained on the supposition, that the undulations of light, in 

 passing through the calcareous spar, assume a spheroidal form ; 

 and this hvpothesis, though it does not apply with the same sim- 

 plicity as the former, yet admits of such precision, that a pro- 

 portion of the axes of the spheroids may be assigned, which will 

 account for the precise quantity of the extraordinary refraction, 

 and for all the phaenomena dependent on it, which Huygens had 

 studied with great care, and had reduced to the smallest number 

 of general facts. 



" That these spheroidal undulations actually exist," says the 

 celebrated Playfair, " he would after all be a bold theorist who 

 should affirm;' but that the supposition of their existence is an 

 accurate expression of the phaenomena of double refraction, can- 

 not be doubted. When one enunciates the hypothesis of the 

 spheroidal undulations, he in fact expresses- in a single sentence 

 all the phaenomena of double refraction. The hypothesis is there- 

 fore the means of representing these phaenomena, and the laws 

 which they obey, to the imagination or the understanding; and 

 there is perhaps no theory in optics, and but very iew in natural 

 philosophy, of which more can be said. Theory therefore, in 

 this instance, is merely to he regarded as the expression of a 

 general law, and in that light rtiiink it is considered by La- 

 place." 



Dr. Young has selected from Sir Isaac Newton's various writ- 

 ings, manv passages favourable to the admission of the undula- 

 tory theory of light, or of a luminiferous ether pervading the uni- 



• From Ure's Dictionary of Chemistry. 



Vol.57. No. 276. jMrt<?lS21. ^^ F verse. 



