[ ft6o ] [Book III. 



CHAP. IX. 



OF THE DIFFERENT REFRANGIBILITY 

 OF THE RAYS OF LIGHT, AND OF CO- 

 LOURS . 



Errors and Incwveniencies in optical Injlrumenti from the Refraclion 

 of Light. Newton, while attempting to remedy thefe, difco'vers a 

 ne-M Property in Ligbt. Phenomena and Caufe of Colours. Achro- 

 matic Telefcope. The Eye an achromatic optical Inftrument. Ex- 

 periments on Colours. Caufe of the permanent Colour of opaht 

 Bodies. 



IN the preceding chapters we have feen, that in 

 finding the foci, fome errors naturally arife by re- 

 fraction from every furface whatever, and by reflexion 

 from all fpherical furfaces. When parallel rays arc 

 refracted by a lens (as in Plate XXII. fig. 49.) the far- 

 ther the ray is from its center, the greater will be its 

 deviation from the point F which we called the focus. 

 This deviation M F is called the longitudinal aberra- 

 tion, and F N its latitudinal aberration ; and as we fee 

 on a piece of paper a fmall circle formed by the rays 

 of the fun intercepted by a convex lens, its magni- 

 tude depends on its radius, or the latitudinal aberra- 

 tion FN. This aberration depends on the aperture 

 of the glafs A B, and it does not increafe in the fim- 



* This fubjeft has been partly anticipated in the ift and 4th 

 chapters of this book, in which it was neceflary to give a fuper- 

 ficial account of the difcoveries concerning light, and of the na- 

 ture of refraftion. The reader will, however, have no caufe to re- 

 gret a little repetition, as the fubjeft is not very clear to a learner, 

 and cannot be too forcibly imprcffed on the mind. 



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