METHOD OF EXAMIKING DISPERSIVE POWERS. 95 



duced are counteracted by-refraction at their emergence from 

 the prifm. 



An inftance in which the colours are fo reverfed* may be Inftance : F. 

 feen by application of oil of fafTafras to a prifm of flint glafs. g^*^ ol * 



So high is the difperfive power of this oil, that, in refractions 

 from flint glafs into it, the red rays are refracted more than the 

 violet. 



It muft be obferved that, in this experiment, when the angle 

 of reflection within a triangular prifm exceeds 60°, the angle of 

 emergence is fuch as would alone occafion the red rays to 

 appear lowermoft ; but, when the glafs ufed is rectangular, the 

 refraction at emergence has an oppofite effect ; any reverfion 

 of colour will therefore be in fome degree corrected, and may 

 not be feen, unlefs the difperfive power of the medium in con- 

 tact much exceeds that of the glafs. 



A cafe of refraction with an inverted order of colours, has Crown glafs and 

 been obferved by Dr. Blair,* in a compound objed-glafs oil of turpentine, 

 where crown-glafs was in contact with oil of turpentine. From 

 trials with lenfes, he likewife inferred, that feveral other fluids 

 have the fame effect, when applied to that glafs. 



With this glafs, and alfo with plate-glafs, I have tried oil of other oils, &c 

 turpentine, and many other fluids that afford a fimilar reverfion 

 of colours, as Iinfeed-oil, olive-oil, the effential oils of berga- 

 mot, iemon, lavender, pennyroyal, and peppermint, ftrong 

 nitric acid, and many artificial compounds that I fhall prefently 

 have occafion to mention. 



The difperfive power of fluor fpar is the leaft of any fub- F' u or fpardif- 

 france yet examined ; fo that, although its refractive power is per cs vcrJ ittr * 

 alfo remarkably low, (confidering its great fpecific gravity,) a 

 prifm of fluor, in contact with water or alcohol, fhows the 

 prifmatic colours to be refracted in an inverted order. 



With heavy fpar, the inflances of reverfion are very mime- alio panderoua 

 rous, as its difperfive power is low, and is accompanied with*?* 1 

 great refractive denfity. In the refractions from this fpar into 

 flint glafs, and into all oils or refins, I believe, without excep- 

 tion, the colours are feen reverfed. 



Rock cryftal likewife difperfes fo little, that it exhibits the and rockcryftah 

 colours reverfed, when it is in contact with many fubftances of 

 lefs refractive power than itfelf. I have tried it with Dutch 

 plate-glafs, with Canada balfam and balfam of capivi, with 



* Edinb. Tranf. Vol. III. 



many 



