1851.) OF THE ROYAL INSTITUTION. 27 
struction of a lens which he derived from two long thin prisms 
placed base to base. He demonstrated that diverging rays of light, 
admitted on one side of such a solid, would issue parallel on the 
other side. 
There are great practical difficulties in fabricating a large glass 
lens. Condorcet and Brewster suggested, and Fresnel effected, 
the construction of a lens of separate prisms, all unnecessary glass 
being removed. Diagrams of such lenses were shown ; and it was 
stated that they were used with a single large lamp placed in the 
focus of the lens. In this position, however, as was shown, all the 
rays which passed above and beneath the lens might escape. To 
intercept the rays Fresnel placed silvered glass mirrors at the 
proper angles. 
Fresnel also made a polygon of straight lenticular prisms pro- 
ducing a long line of strong light: but the greatest improvement 
effected by this great philosopher was the substitution of reflecting 
prisms for mirrors, thus introducing— 
C. The principle of lighting by one Argand lamp, lenses, and re- 
flecting prisms.—Mr. Cowper here demonstrated, by an apparatus 
contrived for the purpose, that when light is incident on the second 
surface of a prism, it may fall so obliquely that the surface cannot 
refract it, and that therefore this incident light is totally reflected 
from the second surface. Thus, if a ray enters the glass prism 
so as to make the angle of incidence greater than 41° 49’ it is 
totally reflected. 
Mr. Cowper showed how this principle is applied in light- 
houses. He stated that the first light of this kind, on a large scale, 
was put up by Alan Stevenson at the Skerrevore; and that, in 1843 
Fresnel tried its illuminating powers against those of mirrors, and 
found the superiority to be in proportion of 140 to 87. 
“On this subject,” said Mr. Cowper in conclusion, ‘‘ one is 
“struck with the intensity and exclusiveness of thought devoted to 
“each part of the whole matter. The Admiralty intensely desire a 
“lighthouse on a particular spot. The Engineer is intensely 
“occupied in surveying, levelling, and building; and with a per- 
“‘ severance almost superhuman, he continues his work during two or 
“three years on the edge of a rock just showing itself above the 
“waves. He makes a temporary barrack on wooden piles on some 
*‘adjacent point. This is all swept away in one night. He builds 
“it again, and is obliged to live in it for fourteen days together, the 
“weather preventing all access to it. Presently, however, a tower 
**138 ft. high stands securely fixed on the exact spot assigned to it. 
“But the Philosopher has also been at work, quietly but intensely 
** considering the laws of reflection and refraction, and has contrived a 
“glass prism of a new form, — without a thought of standing knee- 
“deep in water twelve miles from land. The glass prisms and lamp 
“are now mounted on the tower, and confided to the keepers. These 
“‘men have no careless task. If they have many lamps, as in a 
