74 Count Rumford's Account of a Method of 
these two cylinders project four shadows upon the white paper 
upon the inside of the back part of the box, which I shall 
henceforth call the field of the instrument, two of which 
shadows are in contact precisely in the middle of that field, 
and it is these two alone that are to be attended to. To pre- 
vent the attention being distracted by the presence of unneces- 
sary objects, the two outside shadows are made to disappear, 
which is done by rendering the field of the instrument so 
narrow, that they fall without it upon a blackened surface, 
upon which they are not visible. If the cylinders be each 
3_of an inch in diameter, and 2^ inches in height, as in the 
instrument I have lately constructed, it will be quite sufficient, 
if the field be 2 T 7 ^ inches wide ; and as an unnecessary height 
of the field is not only useless, but disadvantageous, as a large 
surface of white paper not covered by the shadows produces 
too strong- a glare of light, the field ought not to be more 
than T 3 ^ of an inch higher than the tops of the cylinders. 
In order to be able to place the lights with facility and pre- 
cision, a fine black line is drawn through the middle of the 
field from the top to the bottom of it, and another (horizontal) 
line at right angles to it, at the height of the top of the cylin- 
ders. When the tops of the shadows touch this last-men- 
tioned line, the lights are at a proper height ; and when fur- 
ther, the two shadows are in contact with each other in the 
middle of the field, the lights are then in their proper directions. 
In my new-improved instrument (for I have already caused 
four to be constructed) the white paper which forms the field 
is not fastened immediately upon the inside of the back of the 
box, but it is pasted upon a small pane of very fine ground 
glass, and this glass, thus covered, is let down into a groove 
