254
The viewing tube and some of these facetted glasses may have been used in
conjunction with a secret picture or pictures. This effect is described by several authors
of books on “ recreational natural philosophy” , such as Ozanam, Montucla and
Guyot. Charles Hutton translated Montucla’s edition of Ozanam, giving an illustration
of the apparatus and a description of the construction of a “Magic Picture” . Two such
pictures are mentioned: “ Niceron tells us that he executed, at Paris, and deposited in
the library of the Minimes, of the Place Royale, a picture of this kind, which when
seen with the naked eye represented fifteen portraits of Turkish Sultans; but when
viewed through the glass was the portrait of Louis XIII. A picture of Amadeus Vanloo,
much more ingenious, was shewn in the year 1759, in the exhibition room of the
Royal Academy of Painting. To the naked eye, it was an allegorical picture, which
represented the Virtues, with their attributes, properly groupcSj||||t when seen through
the glass, it exhibited the portrait of Louis XV.”
Ebeling (1789) 69, lot 352; 71, lot 370; HuttonMi8o3) ii, 292—298, X I I ; see also Cat.
253-
Inv. 6518, 67/2-4
255 ANAMORPHOSCOPES 3/4 18th C. (316) Fig, 21 jj§
a Prismatic mirror, possibly speculum metal, semi-circular mahogany mourns with
brass knob, semi-cylindrical tin case. Overall height 147, height mirror 117, edge of
facet 22. The four facets form part of a decahedron. A mirror of this type is illustrated
in Musschenbroek (1762) L, fig. 14.
b Prismatic mirror, possibly speculum metal, cylindrical, black-stained soft-wood
mounts with brass finial, cylindrical tin case. Overall height 208, height mirror 123,
edge of facet 23. The four facets form part of a decahedron. Similar to Musschenbroek
(1762) L, fig. 14.
Inv. 72/7
c Semi-cylindrical mirror of speculum metal, semi-circular mahogany mounts with
brass ring at top, semi-cylindrical tin case. Overall height 102, height mirror 63, diameter
42. The Inventory describes this as an English mirror, and its form confirms this.
Whiston (1714) Opticks I, fig. 8, illustrates a cylindrical mirror.
Inv. 7216
d Conical mirror, possibly speculum metal now very dull, conical black-painted tin
case lined with red baize. Height 51, base diameter 65. Illustrated in Musschenbroek
(1762) L, fig. 13.
Inv. 72/g
255 a h e c d I
e Pyramidal mirror with four sides, possibly speculum metal now tarnished, pasteboard
case to fit covered with debprated paper and lined with red baize. Viewing is
from directly over the apex.
Height 55, base 65 x 65. Illustrated in Mussehenbroek f gb z j L, fig. 11; Nollet (1762)
lec. 16, III, fig. 13, 14.
Invy/wm
From the descriptions, it is reasonably certain that at feast four of these mirrors
came from the Ebeling collection. These are: a or b, lot 415; c lot 412 (described
as English work); d lot 414; e lot 416. All the accompanying prints have been lost.
Optical illusions have a perennial interest because of their particular sense of magic.
Instructions for making distorted pictures forErectification” can be found in many
texts of the 17th and early i8th centuries.
Kircher (1671« 16; Ebeling (1789) 78b
256 ICELAND SPAR 4/4 18th C. (406)
A rough cut slab (about 290 x 200 x 80) of Iceland spar, in an oak tray. The double
refraction properties of this crystalline form of calcium carbonate naturally excited
interest, and Benjamin Martin (1704—1782) wrote two essays enthusiastically describing
the properties of “ Island crystal” , and telescope and microscope objectives made from
i% ■
Ma r t in (1774)
257 REFLECTING TELESCOPE: Gregorian pre 1789 (324) Fig. 220, 222
By P. & J. Dollond, London
Signed on the eyepiece mount: DOLLOND LONDON
Length of barrel 860, diameter 128, length of eyepieces 180, 154, height to pivot 540;
speculum diameter 119, focal length 600.
The construction of this telescope is of brass, with primary and secondary mirrors of
speculum metal. The barrel is supported by two swan-necked brackets that attach on
either side of it. The brackets are mounted on top of a conventional stand with a
folding tripod foot. Altitude and azimuth adjustments are by steel worm-screws,
turned, through Hooke universal joints, by brass and mahogany rods. A brass dust
cap fits into the end of the barrel, and the eyepieces may be replaced by a blank for
storage. The sighting telescope is non-adjustable, measures 260 in length with a diameter
of 24, and is fitted with cross-wires. The primary mirror is held against three
lugs by three brass leaf springs fixed to the back of the eyepiece mount, and on this is
scratched the numbers: “ 21/27” , possibly a serial code. The mirror has been cast in a
smooth mould from speculum metal alloy, and its edge is machine finished. It has a