9i 



Method of EXAMtNiNG djs^e^9iVe powers^ 



Tallow, cold - 1,49 

 Iceland {par, wcakeft 1,488 

 Camphor - - 1,487 

 Linfeed oil - - 1,485 

 Butter, cold - 1,480 

 Eflence oflemon - 1,47 6 

 Oil of turpentine, com- 

 mon - - 1,476 



reftifled 1,470 



Oil of almonds — 



olives - 1,469 



peppermint - 1,468 



— «- lavender - 1,467 



Tallow, melted - 1,460 



Alum - 1,457 



Spermaceti, melted - 1,446 



Cryftalline lens of an ox 1,44? 

 to - 1,380 

 Computed average of 



ditto - - 1,430 



Sulphuric acid - 1,435 



Fluor fpar - - - 1,433 

 Nitric acid (fp.gr. 1,48) 1,410 

 Alcohol - - 1,37 

 White of an egg * 1,36 

 ^Ether - - 1,358 



Vitreous humour of an 



eye - - 1,336 



Water - - 1,336 



Atmofpheric air 



(tfauklbee) - 1,00032 



This method is 

 likewife appli- 

 cable to fhew the 

 fUfperfion of 

 light. 



The limit of 

 perfect reflec- 

 tion is marked 

 by a fringe of 

 colours j 



except when 

 (the refractions 

 being unequal) 

 the difperfions 

 are equal. 



If the difperfion 

 by the rarer me- 

 dium begreateft 

 the order of 

 colours w'Ul Be 

 inverted. 



ON THE DISPERSION OF LIGHT. 



The method above defcribed for inveftigating refractive 

 powers, may alfo be employed with fimilar advantage for 

 inquiries into the difperfion of light by different bodies, and the 

 confequences that refult from their combined action. 



When a glafs prifm is placed in contact with water, and 

 brought near the eye, in fuch a pofition that it reflects the light 

 from a window, the extent of perfect reflection is feen to be 

 bounded by a fringe of the prifmatic colours, in the order of 

 their refrangibility.* The violent rays, being in this cafe the 

 moil refrangible, appear ftrongeit. and loweft, on account of 

 the lefs obliquity that is requifite for their reflection. 



But it may happen that two media, which refract unequally 

 at the fame incidence, may difperfe equally at that incidence. 

 Under thefe circumftances, a pencil x>f rays paffing from one 

 of fuch media into the other, will be refracted, without difper- 

 fion of its colours. The boundary of prifmatic reflection would 

 then be found a well defined line, free from colour, if the fur- 

 face at which the reflected light emerges from the prifm, were 

 at right angles to its courfe. 



When the difparity of the difperfive powers of the media is 

 frill greater, it may alfo happen, that the ufual order of prif* 

 matic colours will be reverfed ; and then the red will appear 

 ftrongeft and Joweft in the fringe, unlefs the colours fo pro- 



* Newton's Optics. Book i. part 2. Exp. IS, 



dueed 



