36 On th UNEQUAL 
is only a partial correction of colour; the ss eaicings scabies 
ble rays cannot all be converged to one focus.” 
Tue next method that occurred to me of determining the 
point in queftion was more decifive. This was to obferve whe- 
ther any of this green and purple colour appeared through the 
moft perfect kind of achromatic object-glafs above defcribed, 
and reprefented in the ninth figure, in which there is only one 
refraction. This 1 found to be the cafe; and therefore confi- 
dered myfelf as in poffeffion of fufficient authority for concli- 
ding, that the theory advanced by Mr Doxtonp, and gene-" 
rally received, was defective. For with the large aperture and 
high power made ufe of in thefe experiments, the colour that 
appears in viewing a bright objeé is not weak and hardly fen- 
fible, but a beautiful bright purple inclining to crimfon, and a 
ftrong full green, and thefe in fuch a quantity as evidently to. 
‘be the obftacle to increafing the aperture of the object-glafs.. 
Tus was the conclufion I was then led to, and which I have 
found confirmed by numerous experiments made fince. But 
before entering farther on the fubjeét, it will be neceflary to 
explain what is meant by different mediums not difperfing the 
heterogeneal rays of light proportionally. 
‘ Ler ABvand CD (Figures 14. and 15.) reprefent the fur- 
faces of two mediums, both of which equally refract the mean 
refrangible ray. This we fhall fuppofe to be the green ray, 
though, in this explication, it is not material which is called 
.the mean refrangible ray. ‘The angles of incidence KGL, 
MRN, will then be equal, and the angles of refraction of the 
green ray HGg, PRy, will, alfo be equal in both thefe me- 
diums. 
Let one of thefe mediums CD exceed the other AB fo 
much in difperfive power, as to make the difference of the 
angles of refraction of the green ray, and extreme violet ray, 
in the medium CD, double of what it is in the medium A B3 
that is to fay, the angle vRy double the angle yGg. Then 
¢ if 
