Gtrubb — Graphical Method of Solving Optical Problems. 483 



point of intersection drop a perpendicular on the line a, and the 

 length of this perpendicular on the same scale will be equal to x in 



above formula, or = — ^7,. 

 a + p 



According to this principle, a machine can be constructed in 

 which the various quantities can be read off by scales, without any 

 calculation whatever. 



I hope shortly to be able to exhibit such a machine to the Royal 

 Dublin Society. 



