1839] 



in the Madras Li(ht House. 



279 



mirror he and reflected from it in the directions h d, c that the angle 

 dhe comprehended between the sides of the reflected pyramid abce, 

 will be equal to the angle bac. 



In order, therefore, to ascertain the lateral extent embraced by each 

 pyramid of reflected rays, it would answer the purpose just as well, if 

 instead of projecting and taking the measuremenrs of these rays them- 

 selves, we were to substitute for them the angles which each lamp 

 forms with the two edges of each mirror, viz. if the angle bac, shown 

 in the annexed plan, be substituted for its equal dhe. As this is a much 

 less troublesome operation, and as it renders the figure less complicated, 

 I have accordingly done so ; and the following are the measurements 

 of these angles, tor the three rows, taken separatel}^, as shown in the ac- 

 companying plan, Fig. 13. 



2\ible, showhig the breadth of the angular spaces iUuminated bj/ the 

 refection of the light of each lamp from the different mirrors. 



No. I Tier. 



Of three lamps and 3 reflectors. 

 Lamp. Reflector. Angle, Total. 



1 



a 



22 





V 





14i 





>> 



c 



1^1 



49 



2 



a 



19 







b 



19 





»> 



c- 



19 



57 



3 



a 



121 





>> 



b 



141 





?> 



c 



22 



49 



In each table the numbers refer to the diflferent lamps belonging to 

 the tier, which are numbered from left to right as shown in the plan. 

 The letters a, b, c, refer to the three reflectors, which are taken in the 

 same order. "No. l,a" consequently, refers to the rays proceeding 

 from the left hand lamp, and reflected from the mirror nearest it on the 

 same side. 



From the above measurements it appears, that a reflected image of the 

 centre lamp will be visible throughout 57 degrees of the circumfer- 

 ence of the horizon, and of the other two in 49 degrees each. 



Therefore, if the eifects be equally distributed over the whole illu. 

 minated space, there w' ould be visible from the upper row of lamps and 



