38i 
1879 .] Proceedings of Societies . 
necessary. From experiments by Faraday for the Trinity House, 
in 1836, relative to the penetrative power of lights, through such 
obstructions as fog, mist, &c., and the more recent experiments 
by the French Lighthouse authorities and by the Trinity House, 
with oil and electric lights, it might be assumed that, with the 
atmosphere so impaired for the transmission of light, that the 
oil luminary at its maximum intensity would be fairly visible at 
the fog signal range of two miles, the eleCtric luminary at its 
double Lizard intensity of 16,500 candles would be visible at 
about four miles. Further, that on more frequent occasions, 
when the oil luminary would be visible at about eight miles and 
a-half, the eleCtric light would be visible at the full range of 17 
miles. 
April 22. — Mr. Bateman, President, in the chair. 
The paper read was on “ Dioptric Apparatus in Lighthouses 
for the EleCtric Light,” by Mr. James T. Chance, Assoc. Inst. 
C.E. The author briefly premised that in the Fresnel or dioptric 
system the source of light occupied the central position within a 
structure of glass zones, or annular segments, by which the inci- 
dent rays were condensed and directed on the sea ; and that there 
were two principal kinds of dioptric apparatus, the fixed and the 
revolving. He then proceeded to make some observations con- 
cerning the different optical treatment which a small radiant like 
the eleCtric arc required from that which suited an ordinary 
flame ; in the latter case, as — for it concerned sea-lights — the 
objeCt was not only to parallelise all the rays emanating from 
any point of the luminary, but also to reduce the vertical di- 
vergence due to the height of the flame by increasing the diameter 
of the optical instrument. On the other hand, the smallness of 
the elecffric arc afforded the opportunity of obtaining from the 
dioptric zones or other elements, by suitable generating sections, 
whatever divergence, whether horizontal or vertical, might be 
desired. It was also pointed out that the source of light, in the 
case of the eleCtric arc, could not be entirely depended upon for 
maintaining the same position in relation to the focal horizontal 
plane, and that consequently — since the vertical divergence due 
to the luminary would move upwards or downwards with any 
vertical displacement of the radiant itself — the mariner could not 
be absolutely secured from failing to see the light, unless a spe- 
cial vertical divergence were given by the dioptric apparatus, 
independently of that caused by the size of the eleCtric arc. 
This, however, involved the adoption for this illuminant of a 
dioptric instrument considerably larger than what was originally 
contemplated, so as to reduce materially the luminary divergence, 
and thereby be free to substitute for it, to some extent, a special 
vertical divergence. The author stated that in 1862 he had ex- 
pressed himself in favour of a much larger apparatus than was 
then employed with the eleCtric light at Dungeness. Also that, 
VOL. IX, (N.S.) 2 C 
