36 



ILLUMINATION 



Fig. 18. 



At any place on the earth's surface a line AO 

 drawn pointing to the pole will be parallel to PF, as 

 the pole is infinitely distant. 



If OC be now drawn and TT at right angles to it 

 and tangent to the place, the angle AOT", i.e. the 

 altitude of the 'pole = the angle OCF, i.e. the latitude 

 of the place. 



Now since the sun completes a revolution in 

 24 hours it is evident that a mirror fixed at an 

 angle with the horizon equal to the latitude of the 

 place, will point to the pole, and if made to re- 



