80 Count Rumford’s Account of a Method of 
photometer, and from that point the distances of the lights are 
measured ; the sides of the tables being divided into English 
inches, and a Vernier, showing tenths of inches, being fixed to 
each of the sliding carriages upon which the lights are placed. 
These carriages are so contrived that they can be raised or 
lowered at pleasure, which is absolutely necessary in order that 
the lights may be always of a proper height, namely, that 
they may be in a horizontal line with the tops of the cylinders 
of the photometer. 
The method of ascertaining when the lights are at the pro- 
per height has already been described. 
In order that the two long and narrow tables, or platforms 
just described, upon which the lights move, may remain im- 
moveable in their proper positions, they are both firmly fixed 
to the stand which supports the photometer ; and in order that 
the motion of the carriages which carry the lights may be as 
soft and gentle as possible, they are made to slide upon parallel 
brass wires, 9 inches asunder, about ~ of an inch in diameter, 
and well polished, which are stretched out upon the tables from 
one end to the other. 
The pane of glass covered with white paper, which being 
fixed in a groove in the back of the box, constitutes the verti- 
cal plane upon which the shadows are projected, is si inches 
long, and 3-'- inches wide, as has already been observed ; 
which is much larger than the dimensions assigned above for 
the field ; namely, l-fe inches wide, and 2 inches high. I had 
two objects in view in this arrangement ; first, to render it. 
easier to fix this plane in its proper position ; and secondly, to 
be able to augment occasionally the dimensions of the field. 
