REFRJNGIBILirr of LIGHT. 47 



green rays are the moft refrangible. But as the angle formed 

 by thefe moft and leaft refrangible rays, would be much 

 greater in the concave if the whole refractions were equal, the 

 whole refraction is here reprefented to be precifely that which 

 is requifite for giving the fame inclination of the green rays to 

 the united red and violet rays, which takes place after refrac- 

 tion through the convex. Hence, as thefe refractions are equal 

 and oppofite, they deftroy each others effect. The rays proceed 

 after refraction without any divergency from unequal refrangi- 

 bility ; and the aberration from the fpherical figure being alfo 

 corrected by means of the concave glafs lens, which is more 

 denfe than either of the fluids ; they are converged to the fame 

 point. 



The conftruction reprefented in thefe figures, is not, how- 

 ever, the moft perfect and convenient for the purpofe. The 

 beft method is to divide the concave glafs neceffary for re- 

 moving the fecondary colour, by making two of the lenfes, or 

 all three of them, concave menifcufes. But throwing the 

 whole concave glafs into one lens, and exhibiting the compound 

 convex and compound concave lenfes feparately, anfwers beft 

 the prefent purpofe of explaining the principle on which the 

 aberration from unequal refrangibility may be totally removed. 

 On the fame account, the difference of the difperfive power of 

 the two fluids, is reprefented greater than it is in reality. 



Having completed an object- glafs of this kind, I carefully 

 examined whether any colour was yet difcernible. For though 

 the red and violet and green rays were now united, it was a 

 thing poffible, that rays of other colours might (till have a fmall 

 inclination to thefe. But I could difcover no colour by the 

 moft rigid teft ; and therefore conclude the refraction of all 

 the rays of the fpectrum to be now equal. If there be any de- 

 viation from this equality of refraction, it is infenfible ; and 

 infenfible errors, in thofe cafes where fenfe is the only judge, 

 may be accounted no errors at all. 



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