REFRANGIBILITY of LIGHT. 49 
concave lens to remove the colour entirely. The colour may be 
totally removed, and the aberration from the figure corrected, 
by a concave which lengthens the focal diftance of the convex 
only one third. ae 
From what hath been explained refpe@ting the total correc- 
tion of colour, it will be underftood, that if the concave 
lengthens the focal diftance beyond what is required, fringes 
of green and purple ought to begin to appear in an inverted 
order. This, which may be ftyled the experimentum crucis in 
this matter, I now had it in my power to try without difficulty. 
The refult turned out exa@ly as I expeGted. Upon applying a 
compound concave, which nearly doubled the length of the 
compound convex, a fringe of green appeared within the focus, 
and a fringe of purple beyond it, which fets the theory of the 
correction of this fecondary colour in the moft fatisfactory 
light. 
THE compound concave in this and all the preceding experi- 
ments, was formed of glafs and an effential oil. 
_I now happened, merely with a view of diverfifying the 
experiment, to apply a compound concave, formed of glafs 
combined with the muriatic acid, which has been mentioned as 
a fluid poffeffing a confiderable degree of difperfive power. 
This opened a new and unexpected fcene. The colours ap- 
‘ peared in the fame order as in the laft experiment, but the 
fringes were fo very broad as greatly to furprife me, and create 
a fufpicion that every thing was not as I had hitherto taken for 
granted. Without delay I included fome of the marine acid 
between two convex lenfes, whofe radii were duly proportioned 
to the difperfive power of that fluid, for the purpofe of cor- 
recting the colour. Upon applying an eye-glafs I found my 
fufpicion verified. The fringes of green and purple appeared 
nearly of the ufual breadth, but in an inverted order, there 
being now a green fringe within the focus, and a purple fringe 
beyond it. Twas the better pleafed at being thus led to’ the 
Vor. II, aye detection 
