 REFRANGIBILITY of LIGHT. 53 
which is oppofite to the refraction produced by the difperfive 
concave. — 
_ ‘Tr 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 {trict- 
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 
‘im the twentieth figure. There are two refractions in the con- 
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 re¢tilineal courfe with the fame equality and 
regularity as in reflection. 
As cuftom has already appropriated the word ea to. 
that kind of refraGtion in which there is only a partial cor- 
rection of colour, in order to avoid confufion, I ‘hall beg per- 
miffion to diftinguifh this entire removal of aberration by the 
term aplanatic*, till a better can be thought of. 
Berore 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 MAardus 
