280 
ning it in the horizontal groove. It may be raised to any 
required height by turning the milled head W, and the height is 
read off on a scale. Mis a sheet of transparent glass, which is 
hinged at its lower end, and may be fixed at any required angle 
with the horizontal plane. In ordinary perspective this angle 
is 45°. The eye # on looking through a small hole in an up- 
right attached to the upper edge of the glass sees by reflexion 
any point P of the Plan transferred to p, which is the per- 
spective of P corresponding to the eye in the position E with 
regard to the given Plan. To obtain the elevation of the 
building, say at a height of 20 feet, we have only got to elevate 
the Plan by turning the milled head W. 
The Perspective of the Plan at any elevation above the 
eye may be obtained by drawing the Plan for the same eleva- 
tion below the eye, and then transposing it, by copying it 
through the sheet of glass held vertical. 
It is manifest by giving various inclinations to the glass 
we may obtain the corresponding phan sections of a cone af 
any form. 
