Dr. Maskelyne’s Attempt , &c. 257 
refrangible rays, and builds thereon an ingenious theory of an 
achromatic objed-glafs, compofed of two menifcus glafles 
with water between them, with the help of an analytical cal- 
culation, Ample and elegant, as his ufually are. 
He has not, however, demonftrated the neceffary exiftence 
of his hypothefis, his arguments for which are more meta- 
phyfical than geometrical ; and, as it was founded on no expe- 
riment, fo thofe made Ance have fhewn its fallacy, and that it 
does not obtain in nature. Moreover, which is rather extra- 
ordinary, it does not account, according to his own ideas, for 
the very phenomenon which Arft fuggefted it to him, namely, 
the great diftindnefs of the human viAon, as was obferved to 
me, many years ago, by the late Mr. John Dollond, 
F. R. S. to whom we are fo much obliged for the invention of 
the achromatic telefcope ; for the refractions at the feveral hu- 
mors of the eye being all made one way, the colours produced 
by the Arft refradion will be increafed at the two fubfequent 
ones inftead of being correded, whether we make ufe of 
Newton’s or Euler’s law of refradion of the differently 
refrangible rays. 
Thus Euler produced an hypothetical principle, neither At 
'for rendering a telefcope achromatic, nor to account for the 
diftindnefs of the human viAon ; and the difficulty of recon- 
ciling that diftindnefs with the principle of the different 
refrangibility of light difcovered by Sir Isaac Newton re- 
mains in its full force. 
In order to go to the bottom of this difficulty, as the beft pro- 
bable means of obviating it, I have calculated the refradion s 
of the mean, mod, and leaft refrangible rays at the feveral 
humors of the eye, and thence inferred the diffufion of the 
rays, proceeding from a point in an objed, at their hilling 
upon 
