28 



Sir J. Conroy. 



[Feb. 15, 



high, were screwed to a rectangular board about 15 centims. by 

 10 centims., in the position shown in the figure, and pieces of white 

 paper, 3 centims. by 3 centims., held against the hypotenuse of 

 each of these triangular prisms by india-rubber bands. The whole 

 arrangement was enclosed in a box with three apertures, which was 

 painted, both internally and externally, a dead black, and was placed 

 at the end of the horizontal board. 



The light from the sliding lamp entered by the left-hand aperture, 

 whilst that which fell directly on the paper, or after reflection from 

 the metallic surface, entered to the right-hand one ; the third aperture 

 allowed the papers to be seen by the observer, who was at about 

 6 decims. from it. 



|tk the actual size. 



The goniometer was fixed on the other side of the photometer, with 

 its vertical axis in the prolongation of the median line of the board, 

 and had a vertical stage, to which the reflecting plate could be readily 

 fastened. The second lamp, which, as before mentioned, was carried 

 by a plate attached to one of the arms of the goniometer, was always 

 placed with its flame edgewise, i.e., radially, and the beam of light 

 limited by means of a diaphragm with an aperture 25 millims. high 

 and 5 millims. wide, placed at a distance of 23 centims. from the 

 centre of the flame and between it and the axis of the instrument. 

 The sliding lamp was placed with the flat side of the flame towards the 

 photometer. 



The experiments were made by first turning the goniometer until 

 the light fell on the paper of the photometer ; the position of the 

 sliding lamp was then altered until both papers appeared equally illu- 

 minated, and the distance of the lamp from the central line of the 

 photometer observed. Tour such observations were made, in the first 

 and third the sliding lamp being placed too near the photometer and 

 the distance increased, and in the second and fourth the lamp placed 

 too far off and the distance diminished until the illumination of the 

 two papers becomes equal. 



