KEVIEWS. 
303 
THE HUMAN EYE.* 
I N this little hook Dr. Dudgeon furnishes his readers with an excellent 
popular description of the eye, and of the optical principles by which it 
is enabled to convey to the brain the visual impressions of external objects. 
The whole explanation is very clear and lucid, and the author has availed 
himself of all the more important recent treatises on vision published both 
in this country and on the Continent. Of especial interest are his observa- 
tions on sub-aquatic vision, upon which he has made many personal experi- 
ments which lead him to the conclusion that perfect vision cannot be 
attained under water by the unassisted eye, and he accordingly recommends 
the use of air-spectacles, the construction of which is fully explained, in 
order to counteract the altered conditions of refraction which must affect 
vision under water. 
This, however, is not the only original part of Dr. Dudgeon’s little book. 
The question of the means by which the eye is enabled to adapt itself to 
vision at different distances is one which has always presented considerable 
difficulties, and the observed phenomena, and especially the direct observa- 
tion of the form of the surfaces of the lens in near and distant vision, led 
the author to the conclusion that changes of convexity in that particular 
part of the eye were not the causes of the adaptation of the eye. More- 
over the mechanism for producing them seemed to be wanting. The con- 
sideration of the phenomena presented by the surface of the lens, when the 
eye is directly examined under changed conditions of vision, suggested to 
Dr. Dudgeon another explanation, namely, a shifting of the position or 
twisting of the lens relatively to the axis of the eye, which, he maintains, 
from the form of the two surfaces of the lens, would produce the alterations 
of refractive power necessary to adjust the eye to changes of distance. 
Upon this ingenious hypothesis we shall not venture to offer an opinion, but 
it certainly seems worthy the attention of students. The little book is 
fully illustrated with woodcuts, and is a valuable contribution to our popular 
scientific literature. 
A GEOLOGICAL MAP OE LONDON.! 
T HIS is Stanford’s capital “ Library Map of London,” coloured geologically. 
The scale is six inches to a mile, far larger than that of any other 
geological map of London, the Geological Survey having published only the 
ordinary “ one inch” map. The area embraces on the north Hampstead, 
Highgate, Clapton, and Leyton ; on the east Stratford, Greenwich, and Lee ; 
on the south Beckenham, Anerley, and Merton ; and on the west Wimbledon, 
Putney, Hammersmith, and Willesden. 
The compilation of the geological information has been carefully carried 
out by Mr. Jordan, who has great experience in such work. The map 
shows not only the Chalk and the various Tertiary beds, but also, to quote 
* “The Human Eye: its Optical Construction popularly Explained.” 
By It. E. Dudgeon, M.D. Sm. 8vo. London : Hardwicke & Bogue, 1878. 
t Stanford’s “ Geological Map of London. The geology compiled from 
the Maps, &c., of the Geological Survey by J. B. Jordan ” London, Stan- 
ford. 1878. 
