addition to those for his own observatory. The barrels and framework were constructed
by the court cabinet makers. Herschel’s polishingjig for his 6'l^inch mirrors* together
with other examples of his work, are in the Science Museum London.
In a letter of 7 December, 1789, Ingenhousz told Van Marum of the excellence of
the Herschel mirrors, which was the reason why the other instrument-makers were
jealous ofhim. Van Marum sought permission to buy a Herschel seven-foot télescope
for 100 guineas, and this was granted by the Directors on 18 June, 1790. The actual
price paid was £ 107-2-0. While on his journey to Germany, Van Marum saw a
similar reflector, belonging to von Zach, on 7 July, 1798. He asked von Zach to
have made for him an ocular micrometer to the same pattern as was being used there.
Frédéric Guillaume Fries, who was employed by Van Marum during 1790 and
1791, assisted at observations made with this teles’cope on 29 December, 1790, and
attended to the vertical movement on 11 April, 1791. Fries rendered his account of the
hours spent, four and five respectively, for which he was paid at the rate of eight
stuivers ( f 0.40) per hour.
Van Marum (L & W ii, 1970) 304; Dreyer (1912); Steavenson (1924); Chenakal
( i958)-
Inv. 73/2
361 TRIPLE PRISM 3/4 18th C. r'fgiyju Figf;l| lf
By P. & J. Dollond, London
Face area 38 x 23; case 45 x 3g x 27.
The set consists of three wedge-shaped prisms in a thin brass frame, the outer hinged
to fold to the inner. The outer prisms are made from crown glass, and have a green
tinge. Their angles are 121]J' and 25°. The Éjner prism is made from white, flint glass,
and the angle is 220. The case is of wood, covered outside with black fishskin and
inside with green velvet.
Van Marum bought a set of achromatic prisms on 15 April, 1789, from Jonathan
Cuthbertson of Rotterdam for ƒ 23 (MV). As this was the day after the Van Swieten
auction sale in Rotterdam, it is conceivable that the set of prisms was acquired in a lot
bought by Cuthbertson. The Inventory names Dollond as the maker of the set.
With these prisms it is possible to explain the phenomenon of achromatism, by
showing refraction with varying dispersion. A detailed examination of a very similar
set of prisms in the Utrecht Museum was published by H. Boegehold in 1929.
J. Dollond (1758); Kelley (1808) 50—77; Boegehold (1928) p( 1929) j (1943« Court &
von Rohr (1929).
Inv. 67/7
262 ACHROMATIC TELESCOPE 33//44 1188tthh CC..
By P. & J. Dollond, London
Signed on the eyepiece mount: DOLLOND LONDON
354)
Length of barrel 384, diameter 60, height to pivot 303; objective diameter 56, thickness
10, focal length about 390; cylindrical fishskin covered case, length 428, diameter
65 to 75.
Three straight bars form the folding tripod foot, and at the top of the pillar is a
simple alt-azimuth mounting for the telescope barrel, which is attached by two
knurled screws. The altitude and azimuth adjustments are made by two worm screws
with knurled heads on the shafts. The objective lens is made up of three components,
the outer both being of glass giving a markedly green colour when viewed sideways.
The construction is almost certainly that of a double-concave flint glass cemented to
two double-convex lenses of crown glass, as announced by Peter Dollond in 1765.
The eyepiece on the instrument is a Huygenian astronomical one, composed of two
plano-convex lenses, of diameter 2 and 7. At the end is an aperture of 1 mm, and a
red glass shade. Focusing is by a screw on the right of the eyepiece.
The museum records register an astronomical eyepiece, no. 354/1, which is now
on the telescope. Originally, there was also a terrestrial, or erecting, eyepiece, and
on 13 March, 1945, it was noted as having a lens missing.
Permission was granted on 31 October, 1788, to buy a 2*/2-foot achromatic telescope
from Dollond, according to the Directors’ minutes, and it was ordered by Van Marum
in a letter of 2 December, 1788, to Cavallo in London. When Van Marum returned
home from a journey on 4 August, 1789, he found that there had arrived: “de 2 '/2 vt.
Achromatische Kijker van Dollond door Mr. Cavallo” (MV). This instrument is
similarly listed in the Inventory, and is the only refractor by Dollond. The purchase
price is recorded in 1789 as ƒ 124-11-8, which is equivalent to £ 11-6-6.
There is, however, an inconsistency in the focal length of the extant telescope, which
is just half that recorded by Van Marum. The instrument could, therefore, have
been purchased at the Ebeling: sale in June 1791, where lot 389 was an achromatic
telescope by DolloncHwith an aperture of 2 ‘/4 inch, and focal length of about
15 inch, contained in a shagreen case.
J. DollondHi 758); P. Dollond (1765) ; Ebeling (1789) 74.
Inv. 70j 1
263 ACHROMATIC TELESCOPE 1770 (351) Fig. 221
By Jan & Harmanus van Deijl, Amsterdam
Signed around the barrel at objective end: Jan van Deijl en Joon Inv. et Fee.
Amsterdam 1770
Length of barrel and eyepiece when extended 265, diameter of barrel 36, height of
stand to pivot 285; objective diameter 25, focal length about 170.
The brass barrel is screwed into a supporting ring that is mounted on a stand by a
compass joint. Focusing is by turning a ring that operates on a thread cut around
the eyepiece tube. The dust cap over the objective screws into place. The stand is
most probably not original, as it closely resembles stands on other instruments and
prisms signed by Duboscq of Paris, who was working under his own name, c. i860.
A very similar telescope, signed as above and dated 1775, has a conventional tripod
Standlftt is to be found in the Scheepvaart Museum, Amsterdam, inv. no. A 3950/V.9.
The objective gives a particularly good image, rather better than the present state of
the Dollond telescope, Cat. 262. The eyepiece fitted to the instrument is a terrestrial
one. The eye-lens is double-convex, diameter 6, the next plano-convex, then,