276 
Communications made to the Society : 
On the form suggested by M. Tresca, and adopted by 
the Commission Internationale du systéme métrique, 
for the Metres Internationaux. By Prof. Miuuer. 
The instrument was described. 
(1) On Methods of drawing in Perspectiwe. By 
Mr J. C. W. Extis. 
(2) On a Method of Levelling (communicated by 
Mr Enuts) proposed by Mr W. H. Srantey. 
1. On methods of drawing in Perspective. 
The problem attempted to be solved was this: ‘Given the 
Plan of a building drawn to scale and in any given position 
with regard to the eye, to cause a pencil by a mechanical 
arrangement to trace out the corresponding Perspective of the 
Plan, whilst the operator causes another pencil to follow the 
outline of the Plan.’ 
The Mechanical difficulties due to friction, jamming &c. 
were such, that no satisfactory result was obtainable by the 
methods employed. 
The following step-by-step methods might however be of 
practical utility, especially in the case of complicated curves. 
The Perspective of any object with regard to any given 
position of the eye is obtained by joining the eye with every 
point of the object and cutting the cone so formed by a vertical 
plane. We draw,to scale the Plan (whose perspective we re- 
quire) on a horizontal plane. We assume the eye to be ata 
height HL above this plane. We take any vertical plane be- 
tween the plan and the eye, at a perpendicular distance SH 
from the eye, so that if S be the position of the eye, FH is 
its orthogonal projection on the vertical plane. 
