Proceedings of the Royal Society. 261 
the human optic nerves; for in the sturgeon, for instance, the eyes 
are placed so exactly back to back, that there are no corresponding 
points of vision requiring to be supplied with fibres from the same 
nerve. In this animal an injury to the left thalamus might be ex- 
pected to occasion entire blindness to the right eye alone; in our- 
selves a similar injury would occasion blindness to all objects 
situated to our right, owing to insensibility of the left half of the 
retina of both eyes. Dr. Wollaston states some other facts, illus 
trating his view of this peculiar distribution of the human optic 
nerves, remarking that in common vision also the sympathy of cor- 
responding points, which receive similar impressions from the same 
object, is dependent upon the same arrangement of nerves, to 
which the term semi-decussation may be applied. In conclusion, 
he observes that, so long as our consideration of the functions 
of a pair of eyes is confined to the performance of healthy eyes in 
common vision, when we remark that only one impression is made 
upon the mind, though two images are formed on corresponding 
parts of the retina, we may rest satisfied in ascribing the apparent 
unity of the impression to habitual sympathy of the parts. But 
when we regard sympathy as arising from structure, and depend- 
ent on connexion of nervous fibres, we therein see a distinct origin 
of that habit, and have presented to us a manifest cause why in- 
fants first begin to give the corresponding direction to their eyes ; 
and clearly gain a step in the solution, if not a full explanation, of 
the long agitated question of single vision with two eyes. 
Thursday, February 26. 
A Paper was read, entitled 
Experimental Inquiries relative to the Distribution and Changes of 
the Magnetic Intensity in Ships of War. By Geo. Harvey, Esq. 
This paper contains the details of a number of experiments 
made on board several vessels, with a view of determining the in- 
fluence of the iron in the ships upon the compass under different 
circumstances and situations. The instruments used for deter- 
mining the intensity consisted of a magnetized cylindrical bar 
2.5 inches long, and ;%; inch diameter, delicately suspended by 
