124 
NATURE 
[De. 15, 1870 

(Camelina sativa) as a single example of agricultural pro- 
duce. “The cultivation of this plant for the seed would 
repay the farmer; an abundance of chaff would be pro- 
duced which would be of infinite service for horses or for 
manure. Ina grazing country like England, where vast 
sums are annually expended for foreign oil-cake, the Gold 
of Pleasure will soon be found an excellent substitute 
under manufacture, and, consequently, a grower should 
find a good remuneration in cultivating the seed. The 
o:l-cake has been found highly nutritious in the fattening 
of sheep and oxen, as it contains a great portion of muci- 
lage and nitrogenous matter, which combined are found 
very beneficial in developing fat and lean.” The prospects 
of making this a most important agricultural plant are, 
we think, too brightly drawn, considering that it has not 
escaped the notice of English agriculturists. Neverthless, 
a few words of this kind on different products might help 
to promote ex periments on their culture and utility. The 
writer’s aim throughout seems to be a general utilisation 
of vegetable productions, and he very ingeniously finds a 
variety of applrcation for those of the Southern States. 
We do not hesitate to say that a few books of this 
d scription on the economic products of different parts of 
the globe, would make us much better acquainted with the 
true value of the vegetable kingdom than we are at 
present. Joun R. Jackson 
A ‘ventures of a Young Naturalist. By Lucien Biart. 
Edited and adapted by Parker Gillmore. (London : 
S. Low, Son, and Marston, 1870.) 
THIS is a narrative of travel in Mexico, intended espe- 
cially for young people interested in Natural History. 
The party consists of a young jad the hero, his father, 
a Swiss naturalist who does all the moralising, a dog, | 
and one of those half-bred Indians who know everything 
and can do everything, who are such a bore in most 
books of Western travel. Though written in a somewhat 
pedantic style, we have no doubt it will find many ad- 
rairers among our adventure-loving young readers, the 
country described being one of unsurpassed beauty and 
interest. For our own part, we should decidedly object 
to being cross-examined in the following manner before 
being allowed to eat our breakfast. “Do you know the 
family of the animal we are going to have for breakfast ?” 
asked Sumichrast. “ Yes ; it isa Rodent.” ‘“ Well done; | 
but how did you recognise it to beso?” ‘ By the absence 
of canine teeth in its jaws, its large incisors, and its hind- 
legs being longer than its fore-legs.” Especially if the 
lesson were given in such a confused style as this :— | 
‘ The bird belongs to the family of Climbers, that is to 
say, to that order which have two toes in front of their 
claws and two behind, like your great friends the parrots.” 
Sull the young naturalist will find in the book much that 
is interesting and amusing ; and the numerous illustra- 
tions and gorgeous binding will make it an acceptable 
present during the Christmas season. 



LETTERS TO THE EDITOR 
(The Editor does not hold himself responsible for opinions expressed | 
by his Correspondents, No notice is taken of anonymous 
communications. | 
Contribution to the Dioptrics of Vision 
In the course of some experiments in reference to vision under 
water, I have ascertained some facts which I do not remember 
to ha e seen mentioned by writers on optics, and which may per- 
haps interest your readers. 
Every swimmer knows that, however clear the water may 
be, and however distinctly he may see from the bank the 
smallest particle of gravel or weed, the moment he plunges be- 
neath the water all becomes obscure, and he can see the outline 
of no*hing at the bottom or suspended in the water distinctly, but 
only blurred patches of various colours. In my first endeavours | 

| sess peculiar optical advantages. 
to find a remedy for this imperfect vision, I found two ways of 
restoring perfect sight. The one was to surround the eye with 
a watertight box, with a piece of plain glass in front. By this 
means, theeye being in the samecondition as to receiving the 
rays of light through an aérial medium as when we are on land, 
perfect vision is retained beneath the water. The other was, 
allowing the eye to remain exposed to the water, to look through 
aglass lens whose proper focal distance in the air, I found, after 
numerous trials, to behalf an inch. The first method is attended 
by the disadvantages that the glass soon becomes dim from the 
condensation of vapour, and it is difficult to make it fit so 
accurately as to exclude the water ; the second is more conyenient, 
as any optician can construct a pair of spectacles suitable for the 
water, and fitted with lenses of the required focal distance. 
Fishes, cetaceous animals, and seals, see perfectly below the 
water, while man’s vision, unassisted, is of the most imperfect 
character. The eyes of these marine animals differ from those of 
terrestrial vertebrates chiefly in this : the latter have a very convex 
cornea, with a large chamber containing aqueous humour and a 
double convex lens behind; whereas the former have a flat 
cornea, hardly any aqueous humour, and a spherical lens, lying, at 
least in fishes, close behind the transparent membrane which is 
their substitute for a cornea. 
Now, as an optical instrument, the eye of terrestrial verte- 
b:ates—and let us take that of man for an illustration—consists of 
two lenses, one placed behind the other. The anterior lens is 
formed by the aqueous humour, its actual figure being a meniscus, 
one surface being convex the other concave, but both surfaces 
uniting if prolonged. According to Donders, the anterior radius 
of curvature, formed by the cornea, is 8mm., the posterior, formed 
by the front of the crystalline, being 10mm. The posterior lens 
is the crystalline, adouble convex lens, its posterior surface, accord- 
ing to the same authority, having a radius of 6 mm. only. The 
combination of meniscus and double convex lens is known to pos- 
The vitreous humour cannot act 
as a convex lens, its form being that of the concavo-conyex lens, 
whose property is to cause divergence of rays of light; but, 
as it lies in contact with the retina, it cannot even produce this 
effect. It acts, together with the aqueous humour, as a watery 
medium for the suspension of the crystalline. 
What happens when the human eye is immersed in water? A 
transparent lens-shaped body will refract the light in con- 
verging rays, if it?s much denser than the surroundmg medium 
through which the rays of light reach it. A simple experiment 
will prove this. Take two watch-glasses with their concavities 
facing one another ; fill the space between them with water ; this 
will form in air, than which it is so much denser, a lens of power 
proportioned to the convexity, but in water it will not refract 
| the light at all, being of the same density as the light-conducting 
medium. The aqueous humour of the eye being much denser 
than the air, acts as a lens in the atmosphere, but being of 
thesame density as water, when the light is transmitted to 
it through water in contact with the eye, we at once lose the 
use of our anterior lens, and can see nothing distinctly ; because 
the crystalline, which alone now acts as a lens, throws its focus, 
as we shall presently see, beyond the retina. 
How, then, are we to recover perfect vision under water ? 
Obviously, by supplying the loss of our anterior lens by another 
lens of equal power. ‘The focal distance in the air of a water 
eas of the meniscus shape and the dimensions given above may 
be calculated ; it is, in fact, two inches or thereabouts ; but, as 
we have seen, it = 0 in water, Lut, as the refractive power of a 
lens diminishes in proportion as that of the medium through 
which it receives rays of light increases, we find that a glass lens 
when immersed in water has only one-fourth of the refractive 
power it possesses in air. So, in order te supply the loss of our 
anterior lens, we find we must use a glass lens of about half an 
inch focus, which, in water, has a focus of about two inches. I 
need scarcely say, that in the case of a double convex lens of 
dissimilar curves it makes a great difference as regards the"re- 
fractive power whether the lens be wholly immersed in water 
or one or other of the convex surfices only. But I need not 
dwell on this s 1bject at present, 
But it is a clumsy method to sup)’, the lossof a lens of two 
inches focus by one of the high ret.acting power of half an inch. 
Besides, a glass lens of this power is so small that the lateral 
field of vision is of necessity very limited, and it has a further 
disadvantage that we can see nothing with it in the air. I there- 
fore sought for a lens that should be free from these defects. 
As the ocular lens whose place had to be supplied is formed 
2 DG At iy li a elle a Nts 
biti 
