REFRANGIBILITT of LIGHT. 53 



which is oppofite to the refraction produced by the difperfive 

 concave. 



It was formerly obferved, that in the confine of a rare dif- 

 perfive medium, and a denfe indifperfive medium, there may 

 be a fingle refraction, in which all the rays are equally refran- 

 gible ; and it has fince been explained with what limitation 

 this is to be underftood, in confequence of the unproportional 

 difperfion which generally takes place in fuch mediums ; of 

 which I was then ignorant. The explanation which refers to the 

 fecond, third, fourth and fifth diagrams, and to the object- 

 glafs reprefented in the ninth figure, is to be confidered as ftrict- 

 ly juft, when, in the fluid employed, the metallic particles are 

 fo far diminifhed, and the particles of marine acid fo far in- 

 creafed, as to render the refraction of the feveral orders of rays 

 proportional in both mediums. 



I have got an object-glafs of this kind, which is reprefented 

 in the twentieth figure. There are two refractions in the coiir 

 fine of glafs and the fluid, but not the leaft colour whatever. 

 Hence it is manifeft that in the refraction which takes place in 

 the confine of glafs and this fluid, and which, on account of 

 the difference of their denfities, is very confiderable, there is no 

 unequal refrangibility of light. The rays of different colours 

 are bent from their rectilineal courfe with the fame equality and 

 regularity as in reflection. 



As cuftom has already appropriated the word achromatic to 

 that kind of refraction in which there is only a partial cor- 

 rection of colour, in order to avoid confufion, I fhall beg per- 

 million to diftinguifh this entire removal of aberration by the 

 term aplanatic *, till a better can be thought of. 



Before clofing this enquiry concerning the optical proper- 

 ties of tranfparent fubftances, I examined more minutely than, 

 I had done before, the qualities of the other mineral acids. 

 The nitrous acid, when of the fame mean refractive denfity as 



the 



* From the Greek « privative, and the verb flXaver'aw 



