322 
accept the Greek MSS. attributed to Zozimus, Pelagius, 
Olympiodorus, Demecritus, Mary the Jewess, and Sy- 
nesius, as exact evidences of date or knowledge. In re- 
gard to more modern matters we regret to find no account 
of Robert Hooke’s important theory of combustion. We 
are glad to observe that M. Hoefer does not echo the 
Wurtzian aphorism : “ La chimie est une science Francaise, 
elle fut instituée par Lavoisier d’immortelle memoire.” 
More liberally our author says, “ Tout en suivant chacun 
une route différente, trois chimistes ont fondé, vers la fin 
du dix-huitiéme siécle, la chimie moderne: Priestley, 
Scheele, et Lavoisier, un Anglais, un Suédois, et un 
Frangais.” 
We should be glad to see in our own country the his- 
tory of matter and of motion studied side by side with 
the history of languages and of numbers. Prof. Kopp 
lectures on the History of Chemistry in the University of 
Heidelberg, and no doubt his example is followed in other 
of the German universities. M. Hoefer’s work is in many 
ways suitable for use as a text-book; it is cheap, it is 
anything but dull, and whatever the errois of arrange- 
ment may be, it contains a great deal of information. 
G, F. RODWELL 
OUR BOOK SHELF 
An Essay on the Physiology of the Eye. By S. H. 
Salom, (Published by the Author.) 
THAT the study of formal logic is not in itself conducive 
to sound reasoning will be acknowledged by many, but it 
is seldom that the truth of the statement is’so fully illus- 
trated as in the short work before us. The author has 
studied the writings of Hamilton, Mill, Bain, and others, 
and with a creditable enthusiasm endeavours to employ 
the new powers he thinks he has thereby acquired, in 
developing a hypothesis of his own to account for the 
phenomenon of vision more satisfactorily than those 
already accepted. An outline of the arrangement, which 
is partly disguised at first sight by the many technicali- 
ties and circumlocutions employed, will be almost, if not 
quite, sufficient for most ot our readers. Commencing 
with a notion broached by Erasmus Darwin, that visual 
perception ensues from retinal motion derived through 
the motile force of light, the author hopes, “ by turning 
the light of modern histological discovery on Darwin’s 
theory of involuntary animal action, to succeed in con- 
vincing associational psychologists that this theory must 
henceforth be included in the creed of @ fosterioré 
thinkers.” With this as a basis, the doctrine pro- 
mulgated may be thus summarised. The eyeball being 
in a constant state of reflex action on account of 
the light acting dynamically on the retina, the motion 
thus produced exerts in the muscles surrounding the eye 
feelings of muscularity similar to those excited when we 
voluntarily determine ocular direction, and consequently 
without any voluntary effort, we are constantly aware 
of visual space properties. To prove this novel hypo- 
thesis the structure of the retina has to be fully entered 
into, and in a most ingenious manner solid fact is distorted 
to satisfy unsubstan iil theory. Taking a single example 
of the reasoning employed, we find that it is necessary jor 
the theory that the fovea centralis of the retina shou'd 
be elastic ; that it is so is evident from the foilowing con- 
siderations :—“In the copious index of that exhaustive 
anatomical work, ‘Quain’s Anatomy,’ under the heading 
“yellow,’ we find, in addition to ‘yellow spot,’ four sub- 
stances oy, namely— 
NATURE 
[Aug. 21, 1873 
Yellow cartilage, 
» fibres of areolar tissue, 
» ligaments of the vertebrze, 
tissue. 
And on referring to the pages of the book in which these 
subjects are treated, we discover that ¢hey have the com- 
mon property of being elastic.” “From this on one of 
Newton’s rules for philosophising “ we are bound to frame 
the following physiological induction,—a// yellow anato- 
mical substance ts elastic” We can hardly think that the 
author is not attempting to fool us. 
LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his correspondents. No notice is taken of anonymous 
communications. ] 
Atoms and Ether 
ATTEMPTS to dispense, in physics, with the ideas of direct 
attraction and repulsion, however interesting, lead generaily to 
a petitio principi, and I fear Prof. Challis’s view, to which atten- 
tion is called in NATURE of August 7, cannot be received as an 
exception. 
For an ether of which the density can be varied is a substance 
that can be compressed and expanded, and what idea is in our 
minds when we speak of compression and expansion in a really 
continuous substance? Continuity implies space, and space that 
is full. Can space be more than fuil? When we say thata 
fluid is compressible and elastic, do we mean anything else than 
that it is made of parts which can be pushed closer together, 
and which, being so pushed, will push each other back? But 
this is repulsion and action at a distance. We do not alter 
the fact by calling the substance ether, and relieving it from the 
influence of gravitation. 
Is a continuous substance, which is capable of compression, 
conceivable? I think not; or if it is, the conception is at once 
more difficult and more opposed to sensible experience than 
that of attraction and repulsion. 
The substance of a bar of iron is not continuous. If I draw 
one end of it towards me, why do¢s the other end follow ? 
What can be the relation between the movement of my end of 
the bar and the ethereal vibrations which must propel the other 
end and all intermediate parts in the same direction ? 
Liverpool, Aug. 9 ALBERT J. Morr 
Instinct 
Sense of Direction 
THE perusal of the correspondence published in the February 
and March numbers of NATURE now to hand, and als» your 
article on “* Perception and Instinct in the Lower Animals,” in 
the number of March 20, has induced a belief in my mind, that 
I may perhaps be able to contribute some evidence beating upon 
the question at issue ; and also that it may have some value from 
having been obtained from a field of observation not generally 
accessible, and from the fact that cattle and horses in Australia 
are subject to very different conditions to those obtaining in 
England. 
I may commence by stating that the question, whether animals 
have or have not a peculiar power of finding their way from 
place to place, suggested itself to my mind very shortly after I 
first went into the Australian bush, now more than twenty years 
ago. It was not long before I satisfied myself that in many 
horses this faculty was strongly developed, but yet unequally in 
different individuals. I afterwards ascertained that it also 
existed in cattle, 
Not only did I find that horses had extensive memories for 
places, being enabled to recollect a track they had followed some 
time previously, but also to remember the way from one place to 
another where no track existed. I found that not only had 
horses this exact memory, but that they possessed another gift 
which at first appeared to me inexplicable. This was, that when 
r dden through the bush, many horses would never, for a moment, 
as it were, lose the recoliection of home, but ‘bear away” in its 
direction, I remarked this not only in a district with which the 
horse might be acquainted from grazing in it, but also when 
travelling and absent for the day from my camp, and from the 
other horse or horses, the ‘‘ mates” of the one I rode. , 
