ON LIGHT AND COLOURS. 165 



beam, adjusted by the spirit-level, and on it moves the up- 

 right on which a plate A moves by micrometer screw E, so 

 that the distance of A from the rays that pass along C C C 

 after flexion by a plate fixed at one end of the beam, can be 

 ascertained by the scale D. I have experimented with this, 

 but I did not find it so easy to work by as the other ap- 

 paratus. C C C is brought to an exact level by screws not 

 noted in the drawing.* 



* This Tract is from the Phil. Trans, for 1850, Part H. See Note IV. 



