■r* 



?8 Experlmeritt and Inquiries refpeSilng Sound and Light. 



fatlon. Wlien d comes to H, the Impreffion will be, either wholly or partly, reflefted 

 with the fame velocity as it arrived, and EH will be equal to DH ; the angle EIH to DIH 

 or CIF ; and the angle of refleQion to that of incidence. Let FG, Fig. 30, be a refracting 

 furface. The portion of the pulfe IE, which is travelling, 'through the refra£ling medium, 

 will move with a greater or lefs velocity in the fubduplicate ratio of the denfities, and HE 

 will be 10 KI in that ratio. But HE is, to the radius IH, the fine of the angle of refrac- 

 tion ; and KI that of the angle of incidence. This explanation of refra£lion is nearly the 

 fame as that of Euler. The total refiefiion of a ray of light by a refra£ting furface, is 

 explicable in the fam.e manner as its fimple refraSion; HE, Fig. 31, being fo much 

 longer than KI, that the ray firft becomes parallel to FG, and then, having to rctura 

 through an equal divcrfity of media, is refledled in an equal angle. When a ray of light 

 pafles near an infledling body, furrounded, as all bodies are fuppofed to be, with an 

 atmofphere of ether denfer than the ether of the ambient air, the part of the ray neareft 

 the body is retarded, and of courfe the whole ray inflefted towards the body, Fig. 32. 

 The repulfion of infle£led rays has been very ably controverted by Mr. Jordan, the in- 

 genious author of a late publication on the Inflexion of Light. It has already been con- 

 jeftured by Euler, that the colours of light confift in the different frequency of the vibrations 

 of the luminous ether : it does not appear that he has fupported this opinion by any argu- 

 ment ; but it is ftrongly confirmed, by the analogy between the colours of a thin plate and 

 the founds of a feries of organ pipes. The phaenomena of the colours of thin plates require, 

 in the Newtonian fyftem, a very complicated fuppofition, of an ether, anticipating by its 

 motion the velocity of the corpufcles of light, and thus producing the fits of tranfmiffion 

 and refledtion ; and even this fuppofition does not much aflift the explanation. It appears, 

 from the accurate analyfis of the phenomena which Newton Jjas given, and which has by 

 ro means been fuperfede4 by any later obfervations, that the fame colour recurs whenever 

 the thicknefs anfwers to the terms of an arithmetical progreflion. Now this is precifely 

 fimilar to the produdion of the fame found, by means of an uniform Waft, from organ- 

 pipes which are dilFerent multiples of the fame length. Suppofing white light to be a con- 

 tinued impulfe or ftream of luminous ether, it may be conceived to adl on the plates as a 

 blaft of air does on the organ-pipes, and to produce vibrations regulated in frequency by 

 the length of the lines which arc terminated by the two refrading furfaces. It may be 

 objedcd that, to complete the analogy, there ftiould be tubes, to anfwer to the organ- 

 pipes.: but the tube of an organ-pipe is only neceflary to prevent the divergence of the im- 

 preffion, and in light there is little or no tendency to diverge; and indeed, in the cafe of a 

 refonant paflage, the air is not prevented from becoming fonorous by the liberty of lateral 

 motion. It would fecm, that the determination of a portion of the track of a ray of light 

 through any homogeneous ftratum of ether, is fufficient to eftablilh a length as a bafis for 

 colorific vibrations. In inflexions, the length of the track of a ray of light through the 

 inflefting atmofphere may determine its vibrations : but, in this cafe, as it is probable that 

 .there is a reflc£lion from every part of the furface of the furrounding atmofphere, con- 

 o tributing 



