266 Perspective Instrument . 
of some service to those who are in the habit of sketching 
landscape, I send you a description of one which I have 
had made, that you may, if you think proper, publish it 
in your Journal. Let AB, fig. 4, plate 8, be a flat rule, 
suppose twelve inches long, having at its extremities two 
arms BD and AF turning upon a joint at B and A; and 
in each arm a circular joint at C and Gr: let the length of 
eaeh arm BD and AF be ten inches : a handle to fix on 
E, and a thread passing through two holes equidistant 
from the handle, making any length, according to the an- 
gle under which the view may be best seen. 
To use the instrument, take the end of the thread in 
the mouth, and hold the instrument upright before the 
eye, then move either or both of the arms till the points 
D and F are brought in a line between the eye and any 
point in the landscape you may wish to delineate ; lay the 
instrument upon the drawing paper, and you will have 
the true situation of such part of the subject. — Proceed 
in like manner, taking care always to keep the base line 
in the same place, till you get as many points as you re- 
quire, by which means any landscape or building may be 
drawn very expeditiously, and w ith a great degree of ac- 
curacy. 
To make the instrument as portable as possible, there 
is a joint in AB, which the handle covers, and the pieces 
DGr and CF are made of thin brass, to fold into the pieces 
AC and BGr ; so that the instrument, when folded up, 
need occupy no more room in the pocket than a small 
spectacle case. 
It is perhaps unnecessary to add that this instrument 
may be used for the purpose of copying, and answers the 
purpose of a triangular or quadrangular compass. I 
am, &c. 
To Mr. Nicholson. 
January 22, 18Q5 
