<cb/>
ing an object through this double prism. For when it
appeared neither raised nor depressed, he was satisfied
that the refractions were equal, and that the emergent
rays were parallel to the incident ones.</p>
<p>Now, according to the prevailing opinion, he observes,
that the object ought to have appeared through
this double prism in its natural colour; for if the difference
of refrangibility had been in all respects equal,
in the two equal refractions, they would have rectified
each other. But this experiment fully proved the fallacy
of the received opinion, by shewing that the divergency
of the light by the glass prism, was almost double
of that by the water; for the image of the object,
though not at all refracted, was yet as much infected
with prismatic colours, as if it had been seen through a
glass wedge only, having its angle of near 30 degrees.</p>
<p>This experiment is the very same with that of Sir
Isaac Newton above-mentioned, not withstanding the result
was so remarkably different. Mr. Dollond plainly
saw however, that if the refracting angle of the watervessel
could have admitted of a sufficient increase, the
divergency of the coloured rays would have been greatly
diminished, or entirely rectified; and that there
would have been a very great refraction without colour,
as he had already produced a great discolouring without
refraction: but the inconveniency of so large an
angle as that of the prismatic vessel must have been, to
bring the light to an equal divergency with that of the
glass prism, whose angle was about 60 degrees, made
it necessary to try some experiments of the same kind
with smaller angles.</p>
<p>Accordingly he procured a wedge of plate-glass,
whose angle was only 9 degrees; and, using it in the
same circumstances, he increased the angle of the water-wedge,
in which it was placed, till the divergency
of the light by the water was equal to that by the
glass; that is, till the image of the object, though considerably
refracted by the excess of the refraction of
the water, appeared nevertheless quite free from any
colours proceeding from the different refrangibility of
the light.</p>
<p>Many conjectures were made as to the cause of so
striking a difference in the results of the same experiment;
but none that gave any great satisfaction, till
lately that it has been shewn to be probably owing to
the nature of the glass then used by Newton. This
conjecture is made by Mr. Peter Dollond, son of John,
the inventor of the achromatic telescope, in a pamphlet
by him lately published in defence of his father's invention,
against the misrepresentations of some persons
who have unjustly attempted to give the invention to
other philosophers, who themselves never imagined that
they had any right to it. After a full and satisfactory
vindication of his father, Mr. P. Dollond then adds,</p>
<p>“I now come to a more agreeable part of this paper,
which is, to endeavour to reconcile the different results of
the 8th experiment of the 2d part of the 1st book of
Newton's Optics, as related by himself, and as it was
found by Dollond, when he tried the same experiment, in
the year 1757. Newton says, that light, as often as by
contrary refractions it is so corrected, that it emergeth
in lines parallel to the incident, continues ever after to
be white. Now Dollond says, when he tried the same
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experiment, and made the mean refraction of the water
equal to that of the glass prism, so that the light emerged
in lines parallel to the incident, he found the divergency
of the light by the glass prism to be nearly
double to what it was by the water prism. The light appeared
to be so evidently coloured, that it was directly
said by some persons, that if Newton had actually tried
the experiment, he must have perceived it to have been
so. Yet who could for a moment doubt the veracity
of such a character? Therefore different conjectures
were made by different persons. Mr. Murdoch in particular
gave a paper to the Royal Society in defence of
Newton; but it was such as very little tended to clear
up the matter. Philos. Trans. vol. 53. pa. 192.—Some
have supposed that Newton made use of water strongly
impregnated with saccharum saturni, because he mentions
sometimes using such water, to increase the refraction,
when he used water prisms instead of glass
prisms. Newton's Opt. pa. 62.—And others have supposed,
that he tried the experiment with so strong a
persuasion in his own mind that the divergency of the
colours was always in the same proportion to the mean
refraction, in all sorts of refracting mediums, that he
did not attend so much to that experiment as he ought
to have done, or as he usually did. None of these suppositions
having appeared at all satissactory, I have
therefore endeavoured to find out the true cause of the
difference, and thereby shew, how the experiment may
be made to agree with Newton's description of it, and
to get rid of those doubts, which have hitherto remained
to be cleared up.</p>
<p>“It is well known, that in Newton's time the English
were not the most famous for making optical instruments:
Telescopes, opera-glasses, &c, were imported
from Italy in great numbers, and particularly from
Venice; where they manufactured a kind of glass which
was much more proper for optical purposes than any
made in England at that time. The glass made at
Venice was nearly of the same refractive quality as our
own crown-glass, but of a much better colour, being
sufficiently clear and transparent for the purpose of
prisms. It is probable that Newton's prisms were made
with this kind of glass; and it appears to be the more
so, because he mentions the specific gravity of common
glass to be to water as 2.58 to 1, Newton's Opt. pa.
247, which nearly answers to the specisic gravity we
sind the Venetian glass generally to have. Having a
very thick plate of this kind of glass, which was presented
to me about 25 years ago by the late professor
Allemand, of Leyden, and which he then informed me
had been made many years; I cut a piece from this
plate of glass to form a prism, which I conceived would
be similar to those made use of by Newton himself. I
have tried the Newtonian experiment with this prism,
and find it answers so nearly to what Newton relates,
that the difference which remains may very easily be
supposed to arise from any little difference which may
and does often happen in the same kind of glass made
at the same place at different times. Now the glass
prism made use of by Dollond to try the same experiment,
was made of English flint-glass, the specific
gravity of which I have never known to be less than
3.22. This difference in the densities of the prisms,
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