MDMD2: His Own Twin Telescope

[Dave Monroe]

"Each of us is to have his own twin Telescope, by Mr. Dollond, fitted
with the latest of his marvellous Achromatics ..." (M&D, Ch. 2, p. 13)

>From Gerard L'E Turner, Scientific Instruments 1500-1900; An Introduction 
(Berkeley: U of California P, 1998 [1980]), "Optical Instruments," pp. 
91-102 ...

"... the first really important optical improvement [in eighteenth century 
scientific instrumentation] was the work of John Dollond, the son of a 
Huguenot weaver, whose hobby was geometry.  In middle life, he joined his 
son Peter in his instrument-making business, and schieved the remarkable 
feat of correcting chromnatic abberation in the telscope by using a 
combination of crown glass and flint glass for the lenses.  The Dollonds 
marketed lens combinations of this type from 1758, for use in telescopes." 
(p. 92)

>From Maurice Damuas, Scientific Instruments of the Seventeenth and 
Eighteenth Centuries, trans. and ed. Mary Holbrook (New York: Praeger, 1972 
[1953]), Part Three, "The Instrument-Making Industry in the Eighteenth 
Century," Ch. 1, "The Evolution of Optical Instruments," pp. 150-72 ...

"The development of optical instruments during the eighteenth century was 
marked by two important events; the perfecting of the reflecting telescope, 
and the making of the first achromatic objectives." (p. 150)

["objectives" = objective lenses = the lenses receiving light at the far 
end, from the eye, of a telescope ...]

"[Leonhard] Euler made a ... positive contribution to the progress of 
applied optics in his celebrated paper published in 1747 in which he
demonstrated the geometrical possibility of making achromatic objectives.  
Since the time of Huygens and Newton theoretical optics
had not progressed any further than applied optics.  In particular, no one 
had re-examined Newton's demonstration of the impossibility of
correcting chromatic abberation in lenses." (p. 153)

"The first achromatic lenses produced in despite [sic] of the Newtonian 
theory passed unnoticed....  However, when achromatic lenses were known and 
in widespread use, it became public that Chester Moor Hall had discovered 
the principle in 1733, and that the optician George Bass had made and sold 
them on his instructions." (p. 153)

"John Dollond opposed Euler with Newton's experiments, and in 1752 there was 
an exchange of letter between Euler and Dollond in which each upheld his own 
view ....  When dollond invoked the authority of Newton, Euler replied by 
quoting the example of the eye in which the dispersion of colours is 
corrected by the juxtaposition of refractive media." (p. 154)

"In 1755, however, a professor at Upsala university, S. Klingerstierna, sent 
to Dollond a geometric demonstration that teh results of Newton's famous 
eighth experiment were false.... it was not until 1757 that [Dollond] 
carried out the simple experiment which he described in his Account of some 
experiments concerning the Refrangibility of Light....  the dispersion of 
light was corrected although the differences in the refractive indices of 
the two media were not compensated for.  It wa this observation which gave 
Dollond the idea of making achromatic lenses." (p. 154)

"... he combined two types of glass where the difference in refractive index 
was greater [than that between glass and water].  He had at his disposal two 
kinds of glass of differing density--a white English crystal glass, a flint 
glass which gave a strong dispersion, and ordinary crown glass, of greenish 
colour, whose dispersive power was weak.  He cut prisms of different angles 
in each glass and found by a process of trial-and-error th combination which 
gave him the desired result." (p. 154)

"On 8 June 1758 [James] Short presented Dollond's first achromatic telescope 
to the Royal Society; it was 5 feet long and gave images of the same clarity 
and luminosity as an ordinary 15-foot telescope.  A report from Dollond was 
read, describing the phases of his research, but he very prudently omitted 
to give any theoretical details which might have permitted other opticians 
to produce achromatic lenses ..." (pp. 154-5)

"He was awarded the highest scientific distinction of the period, the Copley 
Medal, and was admitted to the Royal Society.  In the light of the legal 
proceedings which followed, however, it appears that these honours were 
partly usurped.... several English opticians had already used flint glass 
for producing achromatic objectives before they were patented by Dollond.  
The composition of these lenses may even have been revealed to Dollond in 
1755 by his colleague Robert Rew, but it is probable that Dollond wa better 
placed than any of his compatriots for obtaining sutable pieces of flint 
glass....  John Dollond knew how to profit from these favourable 
circumstances and was the first to make certain of the commercial advantages 
they gave him...." (p. 155)

But wait, there's more ...

"Flint glass is a heavy crystal containing a certain proportion of lead.  It 
is composed of sand, potash or soda and red oxide of lead.  At the time of 
the discovery of achromaticism this type of crystal was obtainable only in 
England.  The authors at the end of the century explain this peculiarity in 
the following manner; the British glass manufactureres began to use coal as 
a result of the deforestation of the industrial regions.... the 
deforestation of England had the cosequence that it assured English 
opticians the exploitation of a remunerative monopoly over a period of half 
a century." (p. 156)

"Foreign customers were served last, and badly served ....  Only glass with 
blemishes was allowed to leave the country." (p. 157)

And, for a brief biography a la Turner above, see Damuas, p. 239.  Daumas' 
text will also undoubtedly prove invaluable as MDMD progresses here, and 
that I've lost days now hosting Chs. 2-3 is all that prevents me from citing 
it at length further and elsewhere here.  And see also, generally ...

Bion, Nicolas.  The Construction and Principle Uses
   of Mathematical Instruments.  Trans. Edmund Stone.
   London: Holland Press, 1972 [1716; trans. 1758].

Okay, now i've hardly consulted every possible souce on Mason and Dixon's 
Transit of Venus expedition, even the ones listed in foreman's article (see 
"MDMD2: Prolegomena") but I've yet to come across a specific reference to 
Dollond's achromatic lenses being used thereupon.  However, if M & D did use 
"two reflecting telescpes made by James Short" (Danson, Drawing the Line, p. 
54), given that Short "Short presented Dollond's first achromatic telescope 
to the Royal Society' (Daumas, Scientific Instruments, p. 154) ... Pynchon 
vs. Danson, 1-1-1?  But do note ...

"The new achromatic lenses invented by John Dollond (1706-1761) also 
received praise from the astronomer royal [here, Nevil Maskelyne, 1767].  
The new compound lens design, made from glasses of different refractive 
index, reduced the annoying rainbow effect common in plain lenses." (Danson, 
Drawing the Line, p. 168)

Even if there's ultimately no certain historical basis for (Pynchon's) 
Mason's remark here, well, fair speculation, at least ...

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