measuring the comparative Intensities of Light. 77 
an inch thick, and of equal height with the cylinder itself, 
and firmly fixed to it from the top to the bottom. This wing 
commonly lies in the middle of the shadow of the cylinder, 
and as long as it remains in that situation it has no effect 
whatever ; but when it is necessary that the diameter of one 
of the shadows be increased, the corresponding cylinder is 
moved about its axis, till the wing just described emerging out 
of the shadow, and intercepting a portion of light, brings the 
shadow projected upon the field of the instrument to be of the 
width or diameter required. In this operation it is always 
necessary to turn the cylinder outwards, or in such a manner 
that the augmentation of the width of the shadow may take 
place on that side of it which is opposite to the shadow cor- 
responding to the other light. The necessity for that pre- 
caution will appear evident to any one who has a just idea of 
the instrument in question, and of the manner of making use 
of it. 
It is by means of these wings attached to the cylinders 
that the widths of the shadows are augmented, so as to fill the 
whole field of the photometer, when the screen with the circular 
opening is made use of. 
As the iower ends of the cylinders which pass through the 
holes made to receive them in the bottom of the box are about 
To of an inch less in diameter than their upper parts, which 
cast the shadows, and as they not only go quite through the 
bottom of the box (which is an inch thick) but project near an 
inch below its inferior surface, and, lastly, as these cylinders 
are not firmly fixed in these holes, it is easy, by taking hold 
of the ends of them which project below the bottom of the 
box, to turn about the cylinders upon their axes, even without 
