266 Defeft in Optical Glaffes, [Book III. 



every objeft would appear of the fame colour, and 

 an irktbme uniformity would prevail over the face of 

 nature. 



We have feen that when rays of light were refrnct- 

 ed by any fur face, the focus after refraction depended 

 on the ratio between the fines of incidence and re- 

 fraction, and that this focus was not a mathematical 

 poinr. The reader will naturally infer, that the aber- 

 ration muft be confiderably increafed, when for each 

 order of colour-making rays a different ratio between 

 thefe fines muft be afflimed. Common obfervers, 

 without underftanding the caufe, may be made fen- 

 fible of this by noticing the colours of objects feen 

 through a telefcope; and from the difficulty of the 

 fubjeft it mignn feem impofiible that any remedy 

 fhould be applied to the inconvenience. Yet who 

 fhall fet bounds to the fagacicy of man ? Mathema- 

 ticians could point out certain combinations of forms 

 and refrangible powers by which the rays might come 

 colourlefs, as the white-making rays are commonly 

 called, to the eye j and a celebrated optician of our 

 times, Mr. Doliond, has had the merit of realiz- 



vironed by a circle, the colours of which, reckoning from the in- 

 ternal part, are blue, white, yellow, reel ; then follows another cir- 

 cular feries, viz. violet, blue, green, 'yellow, red; then purple, 

 blue, green, yellow, red ; green, red ; g.ecoilh blue, red ; greenifh 

 blue, pale red ; greenifli blue, reddiih white. 



Thefe are the colours which appear by reflexion. By the tranf- 

 muted light the following fcri?s are feen. At the center, white, 

 then yeliowlfh red, blaclc, violet, blue, white, yellow, red, Sec. ; 

 fo that the tranfmitted light at any thicknefs, in!L-a J of white, ap- 

 pears of the compounded colour which it ought to have after the 

 fubtraclion of fonie of the conftituent colours by reflexion; after 

 which feries, the colours become too f.iint and diluted to be 

 difcerned. It is curious to obferve, that the glaflb will not come 

 into c.-ntacl without a confiderable degree of prcflurc. Nicbolfon's 

 ^ vol. i. p. 282. 



