'20 



Transactions of the Society. 



But from the concave mirror C D the lines of incidence are found 

 at s and t\ the central ray being the same for both mirrors (Fig. 5). 

 If there be sufficient sky surface uniformly illuminated and nothing 

 intervene between s and t to obstruct the passage of light, it is a 



FiQ, 4. 



Fig. 5. 



matter of indifference whether the incident rays converge at sin, 

 in, or diverge at s' m, t' n. But when anything intervenes to 

 obstruct or occasion loss of light, then the form and position of the 

 mirror surface becomes important, and that particular combination 

 will act best which avoids the impediment, or the loss of light occa- 

 sioned by it. For instance, if dark clouds occupy with bright 

 clouds the expanse required for illumination with the plane mirror, 

 the concave mirror disposed so as to receive rays from the bright 

 cloud and reunite them in p, would give a maximum light ; and 



