20$ Perspective Instrument , Elliptic Instrument . 
and turned to the eye through a side hole in the tube, by 
another mirror C. At the lower end is a pencil E sliding 
in a well -fitted socket, and pressed gently downwards by 
a weight or spring ; or still better, by the hand only. 
The result or use is, that while the images are in succes- 
sion brought into apparent contact with a point in the field 
of view, the pencil may be employed in tracing them in 
true perspective upon the table beneath.* 
Fig. 2, represents a simple rule and string for draw- 
ing ovals on paper. ACE is a silken thread, fixed at 
A, and capable of being lengthened, shortened, and fixed 
by a screw B at the other end. This screw B can be 
placed, by a longitudinal groove in the ruler, at any dis- 
tance from A, and can be made to pinch the thread upon 
any one of the divisions of the rule. At G is a pencil to 
be moved in the bend of the thread. It must be held up- 
right, and it would be easy to contrive means of keeping 
it so ; but it does not seem an object of sufficient necessity 
to add to the price of the instrument. 
In the use, set A at one focus of the intended oval "and 
B at the other. Allow the string to extend till the pencil 
marks the extremity of the conjugate diameter. Draw 
the semi-oval by moving the pencil along in the stretched 
thread : Then reverse the points A and B, placing them 
respectively on the foci occupied before by each other. 
Draw the other semi-oval, which completes the figure. 
I am, &c. 
B, B 
* There is an omission of the grey or rough glass, if the drawing be meant 
fora camera; or of the eye-piece, if it be a telescope. The first focal cor. 
mergence must be made in these, and not at the eye.— Nicholson. 
