of the LigJit emitted by Luminous Bodies. 1 1 



when these united shadows are bounded on three 

 sides by a perfectly white surface, illuminated by the 

 direct rays of two strong lights, — that is to say, when 

 the screen with the quadrangular opening or field is 

 made use of. 



In describing the cylinders by which the shadows 

 are projected, I said they were fixed in the bottom of 

 the box ; but as the diameters of the shadows of the 

 cylinders vary in some small degree, in proportion as 

 the lights are broader or narrower, and as they are 

 brought nearer to or removed farther from the pho- 

 tometer, in order to be able in all cases to bring these 

 shadows to be of the same diameter, which I have 

 found by experience to be advantageous, in order to 

 judge with greater facility and certainty when the 

 shadows are of the same density, I have rendered the 

 cylinders movable about their axes, and have added 

 to each a vertical wing W of an inch wide, tV of an 

 inch thick, and of equal height with tlie 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 situa- 

 tion it has no effect whatever ; but, when it is neces- 

 sary that .the diameter of one of the shadows should 

 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 aug- 

 mentation of the width of the shadow may take place 

 on that side of it which is opposite to the shadow cor- 



