4 
Proceedings of the Poyai Society of Edinburgh. [Bess. 
60 cm. focal length are placed so that a sharp image of the slit or filament 
would be produced at a distance of about 6 m. from the lens. 
The manometer is placed in the beam of light coming from the lens, 
at a distance of about 3 m. from it, in such a manner that the mirrors f 
and g (fig. 2) are both illuminated. In this way two reflected images of 
the slit or filament are produced and are caught on a white cardboard 
screen at a distance of 3 m. from the manometer. The image from the 
fixed mirror g serves to determine the zero position of the manometer. 
In the construction of the manometer the mirrors / and g are adjusted so 
that the two reflected images are as close together as possible. They need 
not coincide exactly, as the zero position can be marked on the screen by 
drawing two fine lines in ink to coincide with corresponding boundaries 
of the fixed and movable images. 
An alteration of the zero distance between the images may be avoided 
by eliminating the following sources of error : — 
(1) An alteration of the relative position of mirrors, lens, slit, and image. 
