12 Experiments on the Relative Intensities 



responding to the other light. The necessity for that 

 precaution 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 cylin- 

 ders 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 lower ends of the cylinders, which pass 

 through the holes made to receive them in the bottom 

 of the box, are about 2V of ^•n 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 opening the box. I 

 said above that the height of the vertical wing at- 

 tached to each of the cylinders was equal to the 

 height of the cylinder itself. This must be under- 

 stood to mean not the total length of the cylinder, 

 comprehending that part of it which passes .into and 

 through the bottom of the box, but merely its height 

 above the bottom of the box, or that part of it which 

 projects above the bottom of the box. 



As it is absolutely necessary that the cylinders 

 should constantly remain precisely perpendicular to 

 the bottom of the box or parallel to each other, it will 

 be best' to construct them of brass, and instead of 

 fixing them immediately to the bottom of the box 

 (which being of wood may warp) to fix them to a 



