Analy sis of Ayscoughs Telescope Adapted to Use at Sea Igor Nesterenko FRIBNSCL Michigan State University East Lansing MI 48824 USA

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Analysis of Ayscough’s Telescope Adapted to Use at Sea

Igor Nesterenko

FRIB/NSCL, Michigan State University, East Lansing, MI 48824, USA

Budker Institute of Nuclear Physics, Novosibirsk, 630090, RUSSIA

(corresponding author, e-mail: nesterenko@frib.msu.edu)

Abstract

An unambiguous accordance between an objective lens for Ayscough′s telescope ′Adapted to Use at Sea′ and

another compound lens which was described by William Eastland in well-known Quere during the trial in

October of 1765 was revealed. Preliminary analysis of small fragments of Quere from Eastland′, Stedman′ and

Champneys′ testimonies was carried.

Introduction

In modern literature on the history of invention and production of achromatic lenses, James Ayscough is

known as optician who gained knowledge about an achromatic compound lenses directly from Chester Moor

Hall. It was before the famous Dollond′s experiments with prisms (1757). Also, he was the first who shared

these knowledge with subcontractors. For instance in 1752, Ayscough engaged the Clerkenwell optician

William Eastland to make several telescopes

with a compound lens consisting of flat-convex crown and flat-

concave flint glasses. According to Eastland's sworn answer, one of the five defendants,

Ayscough wrote out

requirements for making compound lenses.

Quere: Suppose a Crown Glass flat on one side and Convex on the other sufficient to make it 2 foot focus then

with a flint Glass flat on one side and Concave on the other sufficient by adding together make it 3 foot.

At the same time, Ayscough told that the lens had been an invention of Chester Moor Hall Esquire then a

counsellor in the Temple and the two of them went to work during 1753 to construct several of the telescopes

for themselves and for ′friends′.

A few years later, Eastland declared to John Dollond to have made and

publicly sold many such refracting telescopes.

However, using elementary geometric optics, it can be shown that the flint focal length in the compound lens

described in Quere should be -6 feet. The ratio of the crown to flint focal lengths is -2/6, or -1/3 instead of

about -2/3 − the achromaticity condition established by John Dollond from the prism experiments.

In the other

words, this configuration is not an achromatic lens for typical crown and flint combinations of the 18th

century.

Moreover in 1754, Ayscough published advertisement of a refracting telescopes, which from several late

Tryals at Sea by most experienced and competent Judges, are allowed greatly to excel any other yet made in

England.

The detail description of this telescope was published in the same year (Fig.1) and in February issue

of the London Magazine

on the next year. Both descriptions were identical. Since information about this

telescope has been widely disseminated in various sources, including periodicals and even in three languages

(English, French and Spanish). Evidently all practicing opticians in London were aware of this Ayscough′

successful project for Royal Navy.

B. Gee, ′Francis Watkins and the Dollond Telescope Patent Controversy′, (Ashgate, Farnham, 2014), p.94.

Complaint of Peter Dollond versus Francis Watkins, Addison Smith, William Eastland, Christopher Stedman, James Champneys,

1July – 30 November 1765.

′The Answer of William Eastland one of the Defendants to Peter Dollond, Complainant′ (29 October 1765). National Archives,

Kew, London, Public Record Office MSS, Chancery Proceedings, PRO C12/1956/19.

M. Robischon, ′Scientific Instrument Makers in London During the seventeenth and eighteenth Centuries′, (PhD, University of

Michigan, Ann Arbor, 1983), p.289.

D.H. Jaecks, ′An investigation of the eighteenth-century achromatic telescope′, Annals of Science, 67:2, (2010), p.170.

For modern crown BK7 and dense flint SF66 from Schott glass catalogue the achromatic ratio of focal lengths is -0.3254, or

about 2% error from -1/3.

J. Ayscough, ′A Short Account of the Eye and Nature of Vision′, 3rd edition, (London, 1754), following p.26.

The London Magazine, or Gentleman's Monthly Intelligencer, V.24, (1755), p.75 or to see Appendix I.

As a rule, many modern scholars tacitly assume that Ayscough′ telescope adapted for use at sea, had a

compound and most likely achromatic lens. The fact that this was a compound lens easy to understand from

the picture attached to the detail description. But was this lens really achromatic?

Figure 1. The detail description of telescope which correspond to requirements for using at sea: the top picture of

telescope at calm weather; the bottom at windy cloudy weather. The French and Spanish versions of the description have

been cut.

′Adapted to Use at Sea′ Telescope Analysis

It is known, that two different types of this telescope were made. One with a rotating cubic mount where two

singlet objective lenses with short and long focal lengths were installed, and the second with two flip-out

lenses (Fig. 2). The first type has a simplest and more robust design. However, this design is outside a scope of

this article, as it does not include a compound lens version. The design with two flip-out lenses is more flexible

and allows more possible variants including crown-flint compound lenses (Fig. 3). In the version #1 each

lenses are used alternately, all the same as in the design with rotating cubic mount. In the version #2 the second

lens with short focal length is simply split on two positive lenses. Therefore, a noticeable reduction of

chromatic aberration in comparison with version #1 should not be expected. If one of the lenses is made from

crown glass and has a short focal length 𝐹

𝑠, then the additional lens shall have suitable negative focal length to

obtain a compound lens with a long focal length 𝐹𝑙. Obviously, if the negative lens is made from the same

crown glass as the first lens (version #4 and #4a), then a compound lens cannot be achromatic. In this case,

achromatism will be achieved when both lenses have equal and opposite focal lengths or theirs ratio is -1.

JSTOR link to the full version − https://www.jstor.org/stable/community.26290390

Figure 2. Two different types of the telescope ′Adapted to Use at Sea′ with serial numbers 185 (left) and 365(right), and

the same maker signature: ′Ayscough / London / Inv′ et Fecit′, from the collection of Rolf Willach.

Figure 3. Basic crown-forward versions of the design with flip out

lenses, and flint-forward versions in Appendix II are shown. The

greenish lenses are crown glass, and bluish lenses are flint glass.

Here 𝐹𝑠,𝑙 – short and long focal lengths of the telescope; 𝐹

a – focal

length of additional lens; 𝛿 – minimal mechanical distance between

glasses due to the thickness of flip mounts.

It means that the compound lens will have zero optical power.

In general case, when glasses of different types are used, as in

versions #5 and #5a, the achromatic condition is expressed by

following formula:

𝐹

𝑐

𝐹

𝑓

= − 𝜈𝑓

𝜈𝑐

(1)

Here 𝜈𝑐,𝑓 – Abbe numbers of crown and flint glasses, and

𝐹𝑐,𝑓 – theirs focal lengths, respectively.

More detailed analysis of different versions with help of Zemax software can be found in Appendix II.

https://dioptrice.com/telescopes/205 (#185) and https://dioptrice.com/telescopes/242 (#365).

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摘要：

AnalysisofAyscough’sTelescopeAdaptedtoUseatSeaIgorNesterenkoFRIB/NSCL,MichiganStateUniversity,EastLansing,MI48824,USABudkerInstituteofNuclearPhysics,Novosibirsk,630090,RUSSIA(correspondingauthor,e-mail:nesterenko@frib.msu.edu)AbstractAnunambiguousaccordancebetweenanobjectivelensforAyscough′stelescop...

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Analy sis of Ayscoughs Telescope Adapted to Use at Sea Igor Nesterenko FRIBNSCL Michigan State University East Lansing MI 48824 USA

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