ON LIGHT AND COLOURS. 



163 



PLATE XV. 



Is the instrument for ascertaining more nicely the effects of 

 distance on disposition. A is a plate with graduated edge ; it 

 moves vertically on a pivot, and its angle with the horizontal 

 line is measured by the quadrant E. A also moves hori- 

 zontally, and its horizontal angle is measured by the quadrant 

 K. B is another plate with graduated edge, moving in a 

 groove D, by rack and pinion H, and along a graduated beam 

 I. F is a fine micrometer, by which the distance of A above 

 B, when A is horizontal, can always be measured to the 

 greatest nicety by the circle F and the scale G. 



M 2 



