THE ELECTRICAL POLYSCOPE.

HE electrical polyscope is a simple and ingenious apparatus for giving light in the cavities of the human body, the invention of M. Trouvé, who has distinguished himself by the contrivance of several other instruments useful to physicians and involving curious applications of electricity. It

tery, of a reservoir or seconsists of an energetic and constant bat

battery, and of parabolic reflectors adapted to the different uses to which it may be applied, which are furnished with additional mirrors or used without them. A minute platinum thread, connected with the conducting wires of the battery, is placed in the middle of each reflector. When the battery is put in action, the wire becomes incandescent. A special rheostat is provided to regulate the flow of the electricity, which plays a part similar to that of the faucet of a water-reservoir, and controls the flow of the fluid with such exactness as to permit the finest threads of platinum

to approach the point of fusion without passing it. The melting-point of the wires used having been determined in the beginning, can always afterward be avoided without trouble. A galvanometer with two circuits, in which the electro-motive force of the reservoir and that of the battery are in opposition, enables the operator to observe

the condition of the apparatus at every moment. Figs. 1 and 3 represent two of the reflectors. That shown in Fig. 1 is used to light up the mouth, and is of such power as to render the teeth transparent, and make them show every detail of their condition. Placed on the extremity of a probe inserted in the oesophagus, it makes it possible to observe the condition of the stomach. Fig. 3 shows the reflector with mirrors for use in laryngoscopy and rhinoscopy. This adaptation of the instrument may be used by dentists to show the back part of the teeth, without compelling the patient to assume a disagreeable position, as in Fig. 2. The polyscopes are superior to every other device for introducing light to all parts of the human body. With them the source of light may be placed at as minute a distance as is desired from the part to be examined without inconvenience to the operator. With a slight modification the polyscope may be employed as the instrument for performing the very different operation of cauterization. It is of service in other fields than those of medicine and surgery. Captain Manceron, at St. Thomas of Aquinas, has used it to examine the interior of shells and cannon. It is employed likewise in powder magazines, and is a similar apparatus to that used by divers and gatherers of oysters, corals, and pearls, to light up the bottom of the

sea.

MR.

CAPTURE AMONG THE MOLLUSKS.

R. STEPHEN CLOGG has kindly forwarded us a box containing a shanny and a mussel, which he describes as having been taken in the harbor at Looe, Cornwall, in exactly the position represented in the accompanying illustration. The shanny and mussel, our correspondent writes, were taken by a fisherman who was gathering mussels for bait at Looe. Mussels are found in great numbers at the bottom of the harbor there, and the fishermen use a long-handled, four-pronged fork for catching them. A boat is moored over the spot on which the mussels are to be found, and the fork is employed to bring them from below into the boat. In the case in question, our correspondent assures us the shanny and mussel were brought up as shown in our illustration. The fish was alive when taken, and its head firmly fixed in the mussel. This certainly may be considered a curious capture, and from the evidence it may be fairly assumed that the shanny, seeing a tempting mussel with its mouth open, was induced to pop his head in-an operation which Master Mussel doubtless resented by immediately closing its valves, retaining the fish in its deadly grasp. A case in point of fish being taken in this way is mentioned by Couch, in which Lacépède records an instance where, as he (Lacépède) supposes, a shanny had made an attempt to feed on an oyster

that lay with its valves open, in consequence of which it became shut up a prisoner by the closing of the shell. In this case, however, the shanny was more fortunate than the one taken the other day, for it is stated that in this condition of confinement the fish had continued so long that the oyster had been dredged and carried to a considerable

distance. Upon opening it, the captive was again set free alive, and without injury. Shanny are very retentive of life, and would be found nice additions to salt-water aquaria.

In our columns some years ago was recorded an even more extraordinary capture than either of the above, by Mr. Frank Buckland. As doubtless many of our readers have not seen it, we reproduce Mr. Buckland's remarks and the illustration which appeared at the time:

"Some time since, when examining the famous oyster-beds at Helston, near Falmouth, Mr. Fred Hill, of Helston, was kind enough to accompany me and my friend Mr. Howard Fox, of Falmouth, in our expedition. Mr. Hill mentioned to me at the time that he had a curious specimen of a bird that had been caught by an oyster. The bird and oyster had been mounted in a case by Mr. Vingor, of Penzance. I have received from Mr. Hill a photograph of the above event, which I have since had engraved as above. The history is, that a woman who sells oysters went one morning to the Helford River and found the bird -a common rail-dead, with its beak held quite firmly by the oyster, which was still alive.

"The bird in all probability was wandering along the foreshore looking for his dinner, and Mr. Oyster-possibly left longer by the tide than usual-was opening his shells waiting the incoming water. The hungry rail, seeing something that looked like a white and dainty bit of food, pecked at the body of the oyster, and probably pricked him sharply with his beak. The oyster then snapped his shells together as

quick as a rat-trap, and the poor bird instantly became a prisoner to die (or possibly get drowned as the tide rose) in his prison.

"A story is told of a nigger in America who was caught in a somewhat similar manner. The nigger put his tongue between the shells

of a half-opened oyster to suck out the juice, and the oyster caught him tightly by the tongue. Sambo, when released, was chaffed about it. 'Why, the oyster could not have hurt you,' said his friend; 'he has no teeth.' 'No,' said Sambo, 'he 'ave no teeth, but by Gorry he have dam hard gums!""

In the late report on "The Sea-Fisheries " Mr. Buckland published a large mass of information in regard to mussels. In the course of this he says:

"Mussels have a great number of enemies, the chief of which are

VOL. XVII.-8

RAIL CAUGHT BY OYSTER.

five-fingers, or star-fish, and whelk-tingles. It is most interesting to watch the five-fingers eating the mussels. The whelk-tingles, or white buckies, as they are called at Montrose, will clear off in a few hours a large acreage of mussels. The proprietors, therefore, employ women and children to pick them off at low tide. Not only are mussels largely used as bait, but they are a favorite food of the poor, and are sold in large quantities in the streets of the large towns of England-Manchester, Liverpool, Birmingham, etc. They are, in fact, the poor man's oyster.' So much, indeed, are mussels used as food, that a proposition was more than once seriously made to us by the fishermen that it should be illegal to use mussels for human food. As regards their value as food, I have made the following calculation: There are on the average thirty-nine mussels to the pound, equal to 87,360 mussels in a ton. These cost first hand £1 5s. per ton; the cost to the retailers is £3 6s. 8d. per ton. In March, 1876, a large number of crows were observed eating mussels (query, fresh-water) in the Norfolk Broads. There are large quantities of mussels in many of the broads and rivers, especially in South Walsham Broad, and also, I believe, in Hoveton Broad, Ormesby Broad, and Fritton Water. At the present time I believe no use whatever is made of them; it is as well to see if these mussels can not be cultivated and used for bait."-Land and Water.

HOW INSECTS DIRECT THEIR FLIGHT.*

By M. J. DE BELLESME.

THE works of M. Marey have nicely determined the difference be

tween the manner in which birds and insects fly. The bird can change at will the angle of vibration of his wings, and therefore these organs serve to steer his flight. The insect is deprived of this power, because the angle of vibration, as a rule, is invariable in each species, the flying-muscles not being in the wings, but in that part of the thorax which supports the wings.

Knowing these facts, I concluded that if the wing of the insect be merely a motor apparatus, the steering function must be sought for elsewhere; and, from numerous experiments made upon insects of every order, I am convinced that the steering power depends upon the position of the head and thorax, this, in its turn, depending upon the respective positions of the center of gravity and the axis of suspension (l'axe de sustention). Both these elements are sometimes movable, but more often it is the center of gravity which changes.

* From a paper read before the Paris Academy of Sciences, and published in "Comptes Rendus." Translated by M. Howland.