iS39] The Madras Light Houses. 2^3 



in. — An Investigation of the Nature and Optical efficiency of the 

 combination of Mirrors used to augment the illuminating power of 

 the Madra<i Light. — By Captain J. T. Smith, Engineers, F.R.S. 



To the Editor of the Madras Journal of Literature and Science, 

 Sir, 



On looking through some old papers, I have met with, and have the 

 pleasure to send you the accompanying essay, which was written in the 

 year 1833, with a view to publication, bat which from accidental causes 

 was then laid aside. As the subject ha-i more of a local than general 

 interest, and as the evils which are herein pointed out are likely to be 

 soon remedied, I should not now have ventured to request you to give 

 it insertion in your pages, were it not possible that some benefit might 

 result from an exhibition of the defects of a contrivance, which has hi- 

 therto been but little studied, and has been supposed by some to possess 

 considerable merit for its simplicity; at a time when the increasing in- 

 tercourse between the different parts of India, and the urgent demand for 

 the better illumination of our coasts, renders the adoption of a correct 

 theory, and more efficient means, every day more and more desirable. 



The analysis which forms the chief object of this paper, owes its ori- 

 gin to an enquiry in which I was engaged at the time I have mention- 

 ed, some scattered memoranda of which were collected and arranged in 

 the way in which they now stand; and, in order to introduce the subject 

 to the perusal of the general reader, I have thought it necessary to pre- 

 mise a few general observations on the distribution of light, and the 

 nature and objects of Light-house illumination, together wilh a few 

 remarks upon the leading principles by which the latter is governed ; a 

 distinct understanding of them being almost indispensable to a clear 

 explanation of the mode of investigation Mdiioh has been adopted. 



Madras, December 8t h, 1838. 



Preliminary Observations. — Every luminous point whatever throws 

 out rays in all directions, and is equally visible at the same distance in 

 every possible situation. This may be illustrated, by supposing it plac- 

 ed in the centre of a hollow sphere, every portion of the surface of 

 which would receive an equal number of its rays, and be equally illumi- 

 nated by it. 



I shall have occasion hereafter to speak of the upper half of this sphere 

 as the upper hemisphere of rays (Fig. 1 Pl. 3), and the lower half as 



