50 On a Simplified Disperdon-Photometer. 



placed at any elevation. The frame of the tripod-stand is first 

 levelled. A pin at F, directly underneath the centre of the 



mirror, passes through the base of the photometer and fits into 

 a hole in the top of the tripod-stand. The photometer, by 

 turning round this pin, can, without producing any change in 

 the distance of the centre of the mirror from the lamp, and 

 therefore without changing the distance from the screen to 

 the lamp, receive the small horizontal motion necessary for 

 the adjustment of a new inclination of the rays coming 

 from the electric light, without any alteration of the distance 

 of the centre of the mirror from the lamp. The divided circle 

 is clamped with the index at 0°; the lamp is lowered or raised 

 till the illuminated disk formed by the reflected light, passing 

 afterwards through the lens, is in the middle of the paper 

 screen. A little sliding shutter with a fine hole in its centre, 

 seen in the figure, enables a very exact adjustment to be made; 

 but in practice we find that we get sufficient accuracy without 

 the use of the shutter. We now measure the distance from 

 lamp to centre of mirror in inches. Equalizing the intensities 

 of the two shadows by adjusting the lens-slide when looking 

 at them through red or green glass, we now note the lens- and 

 candle-readings ; and we repeat these operations, changing 

 from red to green and green to red about five times in a minute 

 The lamp is now raised or lowered and fixed in any position ; 

 a few seconds suffice to turn the mirror so that it sends its 

 centre ray exactly through the centre of the lens. The dis- 

 tance from screen to mirror in this instrument being 2'2 inches, 

 if $ is the distance from centre of mirror to vertical from lamp, 



