measuring the comparative Intensities of Light. 73 
a name, and call it a photometer. I have likewise made a 
considerable number of new experiments ; but before I proceed 
to give an account of them, it will be necessary to describe 
very particularly the alterations I have found it expedient to 
make in the instruments. 
And, in the first place, the shadows, instead of being thrown 
upon a paper spread out upon the wainscot, or side of the room, 
are now projected upon the inside of the back part of a 
wooden box, 7^ inches wide, io|- inches long, and 3^ inches 
deep, in the clear, open in front to receive the light, and 
painted black on the inside, in every part except the back, 
upon which the white paper is fastened which receives the 
shadows. To the under part of the box is fitted a ball and 
socket, by which it is attached to a stand which supports it ; 
and the top or lid of it is fitted with hinges, in order that the 
box may be laid quite open as often as it is necessary to alter 
any part of the machinery it contains. The front of the box 
is likewise furnished with a falling lid or door, moveable upon 
hinges, by which the box is closed in front when it is not in 
actual use. 
Finding it very inconvenient to compare two shadows pro- 
jected by the same cylinder, as these were either necessarily too 
far from each other to be compared with certainty, or when they 
were nearer they were in part hid from the eye by the cylin- 
der, to remedy this inconvenience, I now make use of two cy- 
linders ; which being fixed perpendicularly in the bottom of 
the box just described, in a line parallel to the back part of it, 
distant from this back inches, and from each other 3 
inches, measuring from the centres of the cylinders ; when the 
two lights made use of in the experiment are properly placed, 
mdccxciv. L 
