of the Light emitted by Luminous Bodies. 9 



middle of the field, from the top to the bottom of it, 

 and another (horizontal) Hne at right angles to it, at 

 the height of the top of the cylinders. When the tops 

 of the shadows touch this last-mentioned line, the 

 lights are at a proper height ; and when, further, the 

 two shadows are in contact with each other in the mid- 

 dle of the field, the lights are then in \\\^\y proper di- 

 rections. 



In my new-improved instrument (for I have already 

 caused four to be constructed), the white paper which 

 forms the field is not fastened immediately upon the 

 inside of the back of the box, but it is pasted upon a 

 small pane of very fine ground glass ; and this glass, 

 thus covered, is let down into a groove made to re- 

 ceive it in the back of the box. This covered glass is 

 51^ inches long, and as wide as the box is deep, viz. 

 31 inches, but the field of the instrument is reduced to 

 its proper size by a screen of black pasteboard inter- 

 posed before the anterior surface of this covered glass, 

 and resting immediately upon it. A hole in this paste- 

 board, in the form of an oblong square, lyV inches 

 wide and 2 inches high, determines the dimensions, 

 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 grooves 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 1% 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^-^ inches, it is evident that the shadows 

 of the lower parts of the cyHnders do not enter the 



