Encyclopaedia Britannica > Volume 8, DIA-England

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D O L
i i;lin(i. 4. An Explanation of an Instrument for Measuring
Small Angles. Phil. Trans. 1754, p. 551. This paper con¬
tains a more detailed theory of the divided object-glass
micrometer, and a testimony of its utility from Mr Short,
founded on actual experiments.
5. An Account of some Experiments concerning the Dif¬
ferent Refrangibility of Light. Phil. Trans. 1758, p. 733.
We have here the important results of a series of accu¬
rate experiments, by which the author had undertaken to
investigate the foundations of the Newtonian theory of
refraction ; though he began them without any hope of a
success so brilliant as that which ultimately crowned his
labours.
It was in the beginning of 1757 that Mr Dollond made
the decisive experiment of putting a common prism of
glass into a prismatic vessel of water, and varying the
angle of the vessel till the mean refraction of the glass
was compensated; when he found that the colours were
by no means destroyed, as they were supposed to have
been in a similar experiment related by Newton ; for the
remaining dispersion was nearly as great as that of a prism
of glass of half the refracting angle. Mr Dollond then
employed a thinner wedge of glass, and found that the
image was colourless when the refraction of the water was
about one fourth greater than that of the glass. He next
attempted to make compound object-glasses by inclosing
water between two lenses; but in this arrangement he
found great inconvenience from the spherical aberration;
so that he was obliged to try the effects of different kinds
of glass, and he fortunately discovered that the refractions
of flint and crown glass were extremely convenient for his
purpose, the image afforded by them being colourless when
the angles were to each other nearly as two to three; and
hence he inferred that a convex lens of crown glass and
a convex one of flint would produce a colourless image
when their focal distances were in the same proportion.
The spherical aberration, where the curvature was so
considerable, still produced some inconvenience ; but hav¬
ing four surfaces capable of variation, he was enabled to
make the aberrations of the two lenses equal; and since
they were in opposite directions, they thus corrected each
other. All these arrangements required great accuracy
of execution for their complete success ; but, in the hands
of the inventor, they produced the most admirable instru¬
ments, and he was singularly fortunate in obtaining a
quantity of glass of more uniform density than has been
since manufactured on so large a scale. He afterwards
made some small Galilean telescopes with triple object-
glasses, and Mr Peter Dollond applied this construction
to the longer telescopes with compound eye-pieces, the
alteration rendering the spherical aberration still more
manageable.
The merits of Mr Dollond’s inventions were promptly
acknowledged on the part of the Royal Society by the
adjudication of the Copleyan medal for the year. In 1761,
he was appointed optician to the king, and was elected a
fellow of the Royal Society; a distinction which is often
obtained on easy terms by those whose situation in life
exempts them from the suspicion of seeking it for any
purpose degrading to science, but which is generally an
object of considerable ambition to persons of mechanical
or commercial occupations.
A considerable share of the credit due to Mr Dollond’s
discoveries has been very erroneously attributed by some
late historians and biographers on the Continent to Leo¬
nard Euler, a mathematician who most assuredly has little
need of the appi’opriation of the merits of others to esta¬
blish his claim to immortality. But in fact the only idea
of Euler that could be said to have furnished any hint to
Mr Dollond, has been shown by the calculations of Dr
D O L 95
Maskelyne, and by the experiments of Dr Thomas Young Dolomieu.
and Dr Wollaston, to have been completely erroneous;
nor did Euler even admit the accuracy of Mr Dollond’s
conclusions after his discovery was made, without con¬
siderable hesitation and scepticism. Mr Klingenstierna
had simply expressed a doubt with respect to the result of
Newton’s experiments, though he by no means suspected
the extent of the error. Mr Peter Dollond has sufficiently
vindicated his father’s claim to complete originality, in a
paper read to the Royal Society in the year 1789; he has
also suggested an explanation of the origin of Newton’s
mistake, b}r stating that there exists a kind of Venetian
glass, of which the dispersive power little exceeds that
of water, whilst its specific gravity nearly approaches to
2*58, which is assigned by Newton to glass in general;
and it certainly seems more probable that some such
circumstance as this was the cause of the error, than that
Newton should, as some have suspected, have mixed ace¬
tate of lead with the water which he used, for an experi¬
ment which was so much more likely to be satisfactory
without it.
Mr Dollond’s appearance was somewhat stern, and his
language was impressive, but his manners were cheerful
and affable. He vras in the habit of attending regularly,
along with his family, the service of the French Protestant
church. He constantly sought his chief amusement in
objects connected with the study of those sciences which
he had so much contributed to improve. Perhaps, indeed,
he pursued them with an application somewhat too in¬
tense ; for on the 30th of November, as he was reading a
new work of Clairaut on the theory of the moon, which
had occupied his whole attention for several hours, he had
an attack of apoplexy, which shortly became fatal. He
left two sons and three daughters. His sons succeeded
to his business ; and the younger dying a few years after¬
wards, his place was filled by a nephewy who assumed the
family name, and who still conducts the establishment with
undiminished respectability and success. (Kelly’s Life of
John Dollond, with an Appendix of all the Papers referred
to, 3d edit. 4to, Lond. 1808.) (l. l.)
DOLOMIEU, Deodatus Guy Silvanus Tancred
de Gratet de, a distinguished mineralogist and geolo¬
gist, son of Francis de Gratet de Dolomieu, and Frances
de Berenger, was born on the 24th of June 1750, in the
province of Dauphine.
Fie was admitted a member of the order of Malta during
his earliest infancy, as if he had been devoted from his
cradle to glory and to misfortune. At eighteen he em¬
barked in one of the galleys belonging to the order, and
soonafterwardsunhappilyfoundhimself under the necessity
of fighting a duel, in which his adversary fell. The laws
condemned him to die, but he received a pardon from
the grand master; it was, however, necessary that it
should be approved by the pope, who for a long time re¬
fused to confirm it, notwithstanding the solicitations of
several European powers in behalf of the offender, until
his consent was at last obtained by the Cardinal Torre-
giani. Dolomieu, in the mean time, was closely imprisoned
in the island for nine months, and this period of solitude
seems to have contributed materially to increase the se-
riousnessvof his character, and to confirm him in a con¬
templative turn of mind.
At the age of twenty-two he went to Metz as an officer
in the regiment of carabineers, in which he had held a
commission for seven years; and he displayed great cou¬
rage and personal activity on occasion of an accidental
conflagration which occurred soon afterwards. His leisure
hours ^were employed in the study of chemistry and natu¬
ral history, with the assistance of Mr Thirion, an apothe¬
cary residing in this city. About the same time he also