measuring the comparative Intensities of Light \ 75 
made to receive it in the back of the box. This covered glass is 
5-f- inches long, and as wide as the box is deep, viz. 3^ inches, 
but the field of the instrument is reduced to its proper size by 
a screen of black pasteboard interposed before the anterior 
surface of this covered glass, and resting immediately upon it. 
A hole in this pasteboard, in the form of an oblong square, 
inches wide, and two inches high, determines the dimensions, 
and forms the boundaries of the field. This screen should be 
large enough to cover the whole inside of the back of the box, 
and it may be fixed in its place by means of grooves in the sides 
of the box, into which it may be made to enter. The position 
of the opening above mentioned is determined by the height 
of the cylinders, the top of it being of an inch higher than 
the tops of the cylinders ; and as the height of it is only two 
inches, while the height of the cylinders is 2— inches, it is 
evident that the shadows of the lower parts of the cylinders 
do not enter the field. No inconvenience arises from that 
circumstance ; on the contrary, several advantages are derived 
from that arrangement. 
Instead of the screen just described, I sometimes make use 
of another, which differs from it only in this, that the hole in it, 
which determines the form and dimensions of the field, in- 
stead of being quadrangular, is round, and 1-^ inches in dia- 
meter. And when this screen is made use of, the shadows 
are increased in width (by means which will hereafter be de- 
scribed), in such a manner as completely to fill the field, ap- 
pearing under the form of two hemispheres, or rather half 
disks, touching each other in a vertical line. The object I 
had in view in reducing the field and the shadows to a circular 
form was this ; I imagined that by diminishing the number 
L 2 
