1839J 



VI the Madras Light House. 



285 



By aiTanging the contents of this same table in another form, by adding 

 together the spaces in which the effects produced an equal, it -will appear 

 that there are 9| degrees in which only 



one image is visible 1 



52 in which 2 



64i „ 3 



23i ...... 4 



6i „ 5 



8 „ 6 



S „ 7 



Or to condense it still more, that out of the whole semi-circumference 

 of 180 degrees, the portion in which 1.2.3 and 4 images of a single light 

 only are visible, amounts to 140°, or 7-9ths ; the remaining 2-9ths be- 

 ing illuminated by 5.6.7 and 8. When it is recollected that, as before 

 shown, the brightness of these imag'^s is nnly one half as vivid as that 

 of the direct unassisted light, and wLen it is also considered, that the 

 total value of this direct light is represented by 12 of these doubly 

 powerful images, visible throughout the whole circumference of the 

 horizon, it will no longer be a matter of surprise that the light in its 

 present condition, although stated in the Almanac to be visible (by 

 w^hich I understand possible to be seen) at a distance of 27 miles from 

 the mast-head of a large ship, is in matter of fact with difficulty dis- 

 cerned at 15 or even 10. 



It might have been expected, that the very great difference 

 above shown to exist in the value of the assistance afforded 

 by the reflecting apparatus, as seen from different points, would 

 have occasioned a material inequality in the brightness of the beam 

 generally, as viewed from a distance in different directions ; which, had 

 it prevailed to any extent, could not fail to have attracted attention; 

 and there is no doubt that such would have obtained, if the whole ef- 

 fect produced by reflection, bore any thing like a large proportion to 

 the whole illuminating power. But it has been before shown, that in 

 the situations where it is most powerful, and even within the very small 

 limits in which 8 reflected images are seen, the actual increase of the 

 refulgent power exclusively due to reflection has no more effect than 

 would be derived from the addition of 4 extra lamps ; or that the share 

 of effect due to the assistance of the reflectors, is even then, only one- 

 fourth of the whole brightness of the visible beam. It is therefore 

 hardly to be expected, that a gradual increase or diminution of power 

 which at its maximum does not exceed 1 -4th, and whose average 



