VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 363 



white paper which forms the field is not fastened immediately on the inside of the 

 back of the box, but it is pasted on a small pane of very fine ground glass, and 

 this glass, thus covered, is let down into a groove made to receive it in the back 

 of the box. This covered glass is 54- 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 on it. A hole in this pasteboard, in the form of 

 an oblong square, 1-jL. inches wide, and 2 inches high, determines the dimen- 

 sions, 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 VV of an inch higher than the tops of the cylinders; 

 and as the height of it is only 2 inches, while the height of the cylinders is 2-i-V 

 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 con- 

 trary, several advantages are derived from that arrangement. Instead of the 

 screen just described, sometimes another is made use of, which differs from it 

 only in this, that the hole in it, which determines the form and dimensions of 

 the field, instead of being quadrangular, is round, and l-£^ inches in diameter. 

 And when this screen is used, the shadows are increased in width, in such a 

 manner as completely to fill the field, appearing under the form of 2 hemispheres, 

 or rather half discs, touching each other in a vertical line. 



In describing the cylinders by which the shadows are projected, it was 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, in order to judge with greater facility and certainty when the shadows 

 are of the same density, the cylinders are moveable about their axes, and to each 

 is added a vertical wing -L-l of an inch wide, -J^ of an inch thick, and of equal 

 height with the cylinder itself, and firmly fixed to it from the top to the bottom. 

 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 used. 



As the lower ends of the cylinders, which pass through the holes made to 

 receive them in the bottom of the box, are about ^ 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 



3 A 2 



