ppears, though contrary to the opinion of Newton, and
most other optical writers, that different kinds of matter differ
extremely with respect to the divergency of colour produced by equal
refractions; so that a lens may be contrived, composed of media of
different dispersing powers, which will form the image of any object
free of colour; this discovery Mr. Dollond has applied to the
improvement of telescopes, with great success. It is by no means
improbable, that nature has, for the same purpose, placed the
crystalline lens betwen two media of different densities, and,
probably, different dispersing powers, so that an achromatic image,
free from the prismatic colours, will be formed on the retina. Indeed
we find a conjecture of this kind, so long since as Dr. David
Gregory's time, he says, in speaking of the imperfection of
telescopes, "Quod si ob difficultates physicas, in speculis idoneis
torno elaborandis, et poliendis, etiamnum lentibus uti oporteat,
fortassis media diversae densitatis ad lentem objectivam componendam
adhibere utile foret, ut a natura factum observamus in oculo, ubi
crystallinus humor (fere ejusdem cum vitro virtutis ad radios lucis
refringendos) aqueo et vitreo (aquae quoad refractionem hand
absimilibus) conjungitur, ad imaginem quam distincte fieri poterit, a
natura nihil frustra moliente, in oculi fundo depingendam."

In describing the eye, I observed, that the crystalline humour was
not every where of the same consistence, being much more hard and
dense towards its centre, than externally: in the human eye, it is
soft on the edges, and gradually increases in density as it
approaches the centre: the reason of this construction is evident, at
least we know of one use which it will serve; for, from the
principles of optics, it is plain that the rays which fall at a
distance from the axis of the crystalline, by reason of their greater
obliquity, if the humour were of the same density in all its parts,
would be more refracted than those which fall near its axis, so that
they would meet at different distances behind the crystalline humour;
those which pass towards its extremity, nearer, and those near its
axis, at a greater distance, and could not be united at the same
point on the retina, which would render vision indistinct; though the
indistinctness arising from this cause, is only about the 1/5449 part
of that which arises from the different refrangibility of the rays of
light, as Sir Isaac Newt