Ohituary Notices. 
Ixvii 
scattered in landward directions — never, indeed, escaping from the 
lantern, which in that aspect is always constructed of metal or other 
opaque material. It was to save, as far as possible, this intolerable 
waste that Alan Stevenson, in Sanda Island Lighthouse, which was 
lighted in 1850, introduced a large spherical mirror of silvered glass 
of, I believe, 5 feet 10 inches radius, concentric with the flame, so 
as to reflect hack through the flame the light which otherwise would 
have been wasted. It is obvious that rays thus reflected hack will 
proceed forward thereafter /ro??^ the flame, just as if they had been 
emitted directly by it, and thus they will fall on the optical appa- 
ratus in front in such directions only as will enable it to transmit 
them to the seaward horizon. Whether this method of reflecting 
light back through the flame was thus for the first time employed 
in lighthouse engineering, I am not aware ; but Mr Stevenson has 
stated {^Account of the Skerryvore Lighthouse, p. 293) that it had 
been originally suggested by him so far back as 183 4.^' We have 
already seen that this device was also in 1849-50 adopted by Mr 
Thomas Stevenson in the construction of his Catadroptric Holophote, 
of wdiich it forms an essential element. He, however, for the pur- 
pose of returning back light through the flame did not long rest 
contented with ordinary mirror-reflection. Plaving accidentally 
noticed the brilliant specular look of light emerging from a glass 
prism, where it had undergone two internal total reflections, he 
conceived the idea of constructing the spherical mirror in his holo- 
photal apparatus of such prisms ; and, accordingly, having consulted 
me, I assigned their proper form and mode of combining them, so 
as to form a totally reflecting hemisphere {R.S.S.A. Trans., 1850, 
vol. iv. p. 20). But it was only at a considerably later period that 
the first complete catadroptric mirror was constructed for the Com- 
missioners of northern Lights by Mr J. T. Chance, and it was shown 
by them at the London International Exhibition of 1862. For 
this instrument Mr Chance devised a new arrangement of the pris- 
matic zones, which greatly facilitated its construction. Their figures 
were now generated round a vertical instead of a horizontal axis as 
formerly, and they no longer formed a continuous hemisphere, but 
were separated from each other. Having thus devised the means of 
* This was in a Report to the Commissioners of Xorthern Lights, 10th 
December 1834, p. 28. Edinburgh : Printed by Neill & Co., 183.5. 
