of the Light einitted by Luminous Bodies. 41 



DESCRIPTION OF THE PLATES. 



Plate I., Fig. i. This represents a plan, or rather 

 the outlines of a bird's-eye view of the photometer, 

 upon its stand, together with the ends adjoining to 

 the stand of the long and narrow tables on which the 

 carriages run which support the lights : a, b, c, d, e, f, g^ 

 k, i, k, is the plan of the photometer properly so called, 

 w^hich is a box of wood, painted black within and with- 

 out, with two projecting, horizontal, quadrangular tubes, 

 e, f, g, h, and i, k, a, b, through which the light is 

 admitted. The part of the figure which is bounded 

 by the three straight lines g, h, — h^ 2, — and i, k, and 

 the curved line k, g, is merely a projection of the board 

 which forms the bottom of the box. It is of no real 

 use, serving only to give a more elegant form to the 

 instrument. 



Dotted lines drawn through the axes of the two 

 horizontal tubes above-mentioned meet at the surface 

 of a vertical plane consisting of a piece of sheet glass 

 covered with white paper, which plane constitutes the 

 field of the instrument on which the shadows are 

 projected. 



Two small circles through which those dotted lines 

 pass represent the ground plans of the two cylinders 

 of brass, painted black, by which the shadows are 

 thrown on the field of the photometer. On one side 

 of each of these cylinders there is a projecting wing, 

 a plan of which is represented in the figure. 



Each of the small circles which represent the plans 



