2 45 Fig. 2 12
247 PAIR OF PRISM FRAMES 3/4 18th C. 1^332/a, b) Fig. 213
a Overall dimensions, base 171 x 127, height 232.
b Overall dimensions, base 168 x 125, height 228.
The mahogany frames are provided with five notches on either side to carry prisms
horizontally, and two notches and brass catches above and two holes below to carry
prisms vertically. Frame a has provision for a back board, which is missing, while
frame b is complete with its board. In this board are two slides each with a pinhole.
One slide can move vertically through a distance of 55, and the other moves horizontally
through 37. The prism frames follow exactly the design illustrated in ’s Grave-
sandc, and were used in many experiments on light and colours.
’s Gravesande (1748) CX, fig. 7, 8.
Inv. 6611, 2
248 THREE GLASS PRISMS 3/4 18th C. (332/1, 2, 3) Fig. 213
Fquilateral prisms in brass mounts are fitted with a plain rod at one end and a
screwed rod at the other, with a nut for fixing in position in the frames (Cat. 247).
a Area of glass face 91 x 23.5, length in mount 112.
b, c Area of glass face 103 x 31, length in mount 125.
Inv. 66/1, 2
249 PAIR OF PRISMATIC CELLS 3/4 18th C. (336/1,2) l ig. 215
a, b Face area 60 x 65, overall height 80.
Two equilateral prismatic cells made of glass and brass for the study of the refraction
and dispersion of liquids.
’s Gravesande (1748) CXI. lig. 4; CXVI. fig. 1.
Inv. 66Ij
250 THREE PRISMATIC CELLS 3/+ 18th C. (335/c 2, 3) Fig. 215
Right-angled prismatic cell, sides 38, 38, 52, length of face 111, with mount 135.
Equilateral prismatic cell, sides 51, length of face 117, with mount 135.
c Irregular prismatic cell, sides 39, 51, 57, length of face 114, with mount 137.
The cells have glass sides and brass mounts.
The mounts are fitted with a plug at one end that is removed by a square-ended
spanner. The shaft of the plug continues to a rounded end so that the cell may be
pivoted in the supporting pillars.
d Support: mahogany base (270 x 56) with turned brass pillars (height 105).
T hrough one pillar slides a brass rod with a coupling to engage on the prism spindles.
The position of this rod can be fixed by a locking screw.
Inv. 66/6
251 DOUBLE PRISM OF GLASS !/4 i8 th C . (333) Fig. 214
Area of glass face 106 x 32, length in mount 132 ; support base 196 x 70, height to top
of pillars 108.
Two right-angled glass prisms are mounted with their widest faces in contact, pressed
together by brass strips inside the mounts that are moved by external screws. The
combination rests in its mahogany support by a plain and a screwed rod, the latter
bearing a tightening nut.