44 Experiments on the Relative Intensities 



supported. This circular table, through the centre of 

 which the pillar of the photometer passes, is supported 

 on a strong flange or shoulder in the pillar which is 

 made for it to rest upon. 



The box of the photometer is fixed to its stand or 

 pillar by means of a ball and socket. In the Fig. 2, 

 this box is represented shut up by three sliding wooden 

 doors, a, b, and c. Through the door-way on the left 

 at a, and through that on the right at b, light is ad- 

 mitted into the photometer; and that in the middle, 

 at c, is opened in order to observe the shadows cast on 

 the field of the instrument. 



The places occupied by these three sliding-doors in 

 the ground plan of the photometer (see Fig. i) are as 

 follows. The first (a) fills the opening from a to k; 

 the second (b] that from /to g ; and the third (c) that 

 from i to //. 



Plate III., Fig. 3. This figure represents a plan, or 

 rather the outlines of a bird's-eye view of the whole of 

 the apparatus, drawn to a small scale. 



a is the box of the photometer, which is represented 

 as being closed above with its lid or wooden cover. 



b and c are the two sliding carriages on which the 

 lights are placed, which are the subjects of the experi- 

 ments. There is a movable stage or platform belong- 

 ing to each of these carriages, which, by means which 

 will presently be described, can be placed higher or 

 lower. It is upon these platforms, and not on the 

 bottoms of the carriages, that the lights are placed ; 

 and, as they are movable upwards and downwards, the 

 lights to be compared can easily be placed exactly at 

 the same height, which is always necessary. Each of 

 the pieces of board which form these platforms has 



