Perspective, Instrument , 'Elliptic Instrument . 201 
To make your drawing larger, pull out the tube of the 
pillar C, fig. 3, and fix it with the screw e ; then pull out 
the sliding tubes H, till the pencil is within half an inch 
of the paper, (in the middle of the board,) and proceed 
as before. 
To make the drawing smaller, shorten the tubes C and 
H by sliding them in, and proceed as before. 
No. 10. 
Description of an Instrument for drawing in true Per- 
spective from Nature , and of another of considerable 
Simplicity and Cheapness for delineating Ovals . By 
1 1 . B.* 
[With an engraving’.) 
To Mr . Nicholson , 
•SIR — -As I observe that you are willing, in your ea 
paoity of Journalist, to lay before the public any sketch 
or outline of invention that may promise to be useful, 
whether in its ultimate state of improvement or not, 1 am 
encouraged now and then to send my thoughts, queries, 
observations, or news, as they may occur. The follow- 
ing instruments are offered to your notice, in hopes they 
may appear in your excellent collection. 
Fig. 1 , Plate 6 , is a sketch of an instrument for per- 
spective, made some years ago by Holland, and of which 
I know not the inventor. A telescope or camera is sus- 
pended vertically on a frame by an universal joint or 
jimbals. Horizontal rays A, are directed down the tube 
by a plane mirror B, and are again rendered horizontal, 
Yol. il 
* Nicholson, voi. 9, p. 122, 
b b 
