color when first taken, then turns red, and lastly black; is one of the best of the southern table fishes; weight, from four to sixteen pounds.

Crab-cater, Sergeant fish (Elacate Atlantica Cuv.). Family of Scombridæ, or mackerels; found along the shores of the inlets, where it lurks for prey among the mangrove roots; very voracious; takes clams or mullet bait; color, silvery, with a black stripe along the sides; hence its local name of Sergeant fish; the under jaw longer than the upper; weight up to twenty pounds; a good table fish, though inferior to the former.

Whiting or King-fish (Umbrina alburnus DeKay). Shaped like a perch, double dorsal with strong spines; color, gray and black above, yellowish white beneath; mouth and teeth small; bottom fish of deep water; takes clam bait; very good table fish; weight, from one to two pounds.

Croker (Micropogon undulatus Cuv.). A southern fish of the perch family; in form, deep like the sheepshead; color, silvery; takes clam bait eagerly; weight, from one to two pounds; a good table fish.

Hog-fish, Sailor's Choice (Hamulon fulvomaculatum Mitchell). Shaped like the last; a good pan fish; weight, from half a pound to a pound; takes clam bait on the bottom.

Cat-fish, of the salt-water (Galeichthys marinus DeKay). Handsomer in form and color than the fresh-water cat; has a forked tail and very high dorsal fin; takes fish or clam bait on the bottom; weight, 10 to 15 pounds,

Black trout (Grystes salmoides Lacepède). This is a fresh-water fish of the perch family, much resembling in appearance and habits the black bass of the western waters, except that it has a larger head and mouth, and grows to a larger size, say to twelve or fifteen pounds. It takes live bait, spoon or bob, which is a bunch of colored feathers with three hooks concealed among them.

Besides the above fishes, these waters contain blue fish, Spanish mackerel, beluga, mullet, Jew fish, drum, shad, lady fish, porpoise, sharks, saw fish, sting ray, the hawk's bill turtle, the soft-shelled turtle, the green turtle, clams, oysters and crabs, of various kinds.- S. C. CLARKE.

GEOLOGY.

DISCOVERY OF LOWER CARBONIFEROUS FOSSILS ON THE RIO TAPAJOS. I am just returning from a very interesting and profitable trip up the Rio Tapajos, where I have had the good luck to discover an extensive set of limestones, sandstones, and shales, of lower carboniferous age, from which I have made a very large collection of beautiful fossils. As near as I can ascertain at present, I have at least one hundred and fifty species of Brachiopods, Lamellibranchs, Polyzoons, Gasteropods, Trilobites, fishes, and a few plants, the majority of the species being determinable. Of the Brachiopods I have some magnificently preserved specimens, showing interiors. I am going back to Pará to give up my little steamer and divide up my party. I then return to the Tapajos with a very small party, including a photographer, to examine more carefully,

not only these rocks, but to study the Amazon sandstones and clays. I have seen nothing to cause me to change my opinion about the age of the last named formation. I have not succeeded in finding any fossils in them. I have found beautiful fossil leaves of apparently recent plants, in a recent ironstone. In the hill of Creré, Monte Alegre, and near Santarem, beds of basalt occur. C. F. HARTT, on board Government Steamer "Jurupensem," near Monte Alegre, Rio Amazonas, Oct. 5th, 1870.

NEW FOSSIL FISHES.-Prof. COPE has recently studied the genus Saurocephalus and allies, from the Cretaceous, and states as a result, that these fishes are not in the least related to the Sphyrænidæ, where they have been placed heretofore. The structure of the mouth is like that of the Characinidæ, while the neural arches are distinct and the tail vertebrated as in Amia. The pectoral spines have been described by Leidy, as those of a Siluroid, under the name of Xiphactinus; and the beautifully segmented rays referred to Ptychodus, by Agassiz, he regards as the anal or caudal rays of Saurocephalus. The affinities might be more correctly expressed as combining characters of Salmo and Amia. Professor Cope describes a new genus, Ichthyodectes, type species I. ctenodon; the former differs from the known genera, Saurocephalus and Saurodon, in not having the series of nutritious foramina on the inner side of the alveolar ridges. He refers these fishes to a new family, under the name of Saurodontidæ.

PLASTICITY OF ROCKS. - The old cobble-stone pavement in Waverly Place, between Broadway and Mercer street, being now in process of removal, my attention has been drawn to the forms of the stones, especially the harder ones, quartzites, etc. The coarser granulated paving stones have generally crumbled, but the compact stones have been modified convex surfaces in one case fitting into concave in another; none of them retaining a normal form. Now, although the crown of these stones has been worn by the attrition of constant and heavy travel, no such wear can have taken place on their perpendicular surfaces, and I am therefore convinced that they have been moulded into one another by pressure only. On conversing with the workmen, they all concurred as to the fact, and the foreman stated that his attention had been called to it before. Very probably I am myself only repeating what is already well known to others. GEORGE GIBBS, New York.

SALT PLAINS IN NEW MEXICO. - Brevet Major General August V. Kautz, U. S. Army, writing from Fort Stanton, New Mexico, informs me that there is a valley of some two hundred miles long and twenty wide, lying between the Sierra Blanca and the San Andreas and Occura mountains, in that Territory, in which there is no stream, and only a few alkaline springs and salt lakes, or ponds. Where the road from Fort Stanton to El Paso crosses it, about sixty miles south of that post, is a plain of white sand, apparently granulated gypsum, which has drifted into mounds, forty and fifty feet in height. Water of a strongly alkaline character is obtained by digging a few feet, and around the edges of this district, salt marshes exist, where in the dry seasons, great quantities of almost pure salt may be collected. The sand is so white and the plain so extensive as

to give the effect of snow scenery. As I do not remember to have seen a description of the place in print, I send you this note with a specimen of the sand forwarded by General Kautz. George Gibbs, New York.

MICROSCOPY.

A NEW FORM OF BINOCULAR FOR USE WITH HIGH POWERS OF THE MICROSCOPE.*~~ - Of the several forms of binocular arrangement for the microscope which have hitherto been constructed, only such as are adapted for use with low powers exclusively, have as yet come into general use. Of these, the Wenham prism is the popular favorite, and hardly any other form is employed at all by British or American constructors. Mr. Wenham's binocular, when employed with powers below about onehalf inch, leaves nothing to be desired; but with higher powers than this, the field is so imperfectly and so unequally illuminated that it ceases to be available.

The Wenham binocular, like the original binocular of Dr. Riddell, and like the different forms constructed by Mr. Nachet, divides the light, after it has passed the objective, by a vertical section passing through the middle of the entire bundle of pencils, into two equal portions, one of which is directed to each eye. But although the entire body of the light is thus equally divided, the same is not true of the several pencils which make it up. Only those pencils in fact can undergo equal division whose radiant points in the object lie exactly in the plane of the section. All others will be divided unequally, and the inequality will be greater in proportion as the radiants are more distant from that plane. If the division could be effected at the centre of the front lens of the objective, the inequality just spoken of would disappear; but such a division is of course impracticable. With objectives of low power, the base of each conical pencil of rays (which is the area of the front lens of the system) is so large, that the inequality of illumination consequent upon the unequal division of the pencils themselves is not sufficiently great to be objectionable; but with high power objectives, the pencils are very slender; and at the distance behind the combination at which it is necessary to place the binocular construction, many are very disproportionately divided, and many escape division altogether.

By the introduction of an erector into the body of the microscope, the pencils, which cross each other once in entering the front lens of the combination, may be made to cross a second time; and it is obvious that if the dividing apparatus of the binocular be introduced at the point of this second crossing, all the pencils will be divided with the same equality as they would be if the division could be effected at the centre of the front lens itself. Availing himself of this principle, Mr. Tolles, some years since, constructed a binocular eye-piece which solves completely the optical problem under consideration for all powers; but this instru

Read by F. A. P. Barnard LL. D.. President of Columbia College, N. Y., before the Microscopical Section of the American Association for the Advancement of Science, Troy meeting.

ment is costly, and apart from this objection, it has for some reason or other failed to become a favorite with those who have used it.

It is now two or three years since Mr. Wenham suggested the practicability of constructing a binocular for high powers, by means of a contrivance which should reflect one-half the light of each pencil and transmit the other half. This plan was to take a glass prism with parallel surfaces, and to cut this by an oblique section at an angle suitable to reflect one-half the light which should be incident upon it after entering the prism perpendicularly to one of the original faces. The two portions of the divided prism being replaced in position to reconstruct the original prism, the surfaces of section being very nearly but not quite in contact, the whole is placed behind the objective, when the transmitted portion of the light will give one image, while the reflected portion, after a second reflection within the prism, will furnish the other. In this arrangement there is a possibility of some confusion in the image seen by reflection, in consequence of the duplication of the reflecting surface. On this account, or for some other reason not stated, Mr. Wenham did not follow up his invention.

In the January number of "Silliman's Journal" for 1868, Professor Hamilton L. Smith, now of Hobart College, described a binocular arrangement invented by himself, in which it was proposed to effect the division of the light by means of a long thin glass reflector placed very obliquely in the body of the microscope. As both surfaces of such a mirror will reflect light with intensity, it is necessary that these surfaces should not be parallel. It was Professor Smith's first idea to make the reflecting plate sufficiently wedge-shaped to throw the second image out of the fleld; but experiment showed him that, by making the inclination of the surfaces very slight, the images might be made perfectly to coalesce. This construction involved the disadvantage that the length of the body of the microscope could not be varied, but it was attended with an important saving of light. Hitherto Professor Smith's binocular has not been constructed by regular opticians, and its merits are not fully known. The constructions by Professor Smith himself perform very well, but have a rather limited field.

Messrs. Powell and Lealand, of London, have patented a binocular which resembles Professor Smith's in that it divides the light by reflection at the first surface of a glass mirror; but the surfaces of this mirror are parallel, and the image from the second surface is got rid of by giving to the glass considerable thickness. The reflected rays are reflected a second time by means of a right angled prism. As this arrangement is actually constructed, the image seen by reflection is greatly inferior in brilliancy to that formed by the transmitted rays. In fact, when very high powers are employed, the image by reflection is practically unavailable for any useful purpose. This evil might be remedied by increasing the angle of incidence at which the rays from the objective fall upon the first reflecting surface; but this expedient would be attended AMER. NATURALIST, VOL. IV.

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by a large increase in the amount of light lost at the second reflecting surface, and by a corresponding diminution of the brightness of the image seen by transmission.

Binoculars constructed on the principles of those last described may be called cata-dioptric, in contradistinction from those which split the body of the light geometrically, and which are properly denominated stereotomic. They have not the advantage which belongs to stereotomic binoculars, of presenting the object viewed in all its three dimensions. But they permit what most observers regard as very desirable, or find at least very comfortable, the use of both eyes at the same time. It is true that there are many whom practice has so accustomed to the use of a single eye, that they profess to suffer no inconvenience from this mode of observation, and regard binoculars with indifference except so far as they are recommended by their stereoscopic effect. But however slight may be the momentary inconvenience attendant on observation with a single eye, it is believed that no microscopist can continue to observe in this manner for a series of years, without finding that his eyes have lost the equal power which they once possessed of accommodating themselves to distances. It seems impossible to prevent this result from supervening sooner or later, unless by maintaining a strict impartiality in the employment of the eyes alternately at the microscope; and this is what few remember, or if they remember, are disposed to do. If by the use of a binocular this evil can be prevented, this fact alone is sufficient to make a good form of this instrument adapted to the higher powers desirable. Such a form is believed to have been found in the construction now to be described.

If a rectangular prism of calc spar be cut with four of its faces parallel and the other two perpendicular to the direction of the optic axis, a ray of light incident perpendicularly upon any one of the lateral faces will be divided by double refraction into two rays, but both of these two rays will pursue the common direction of the incident ray continued. There is a large difference between the two indexes of refraction. The index of the ordinary ray is 1.6543, and that of the extraordinary, 1.4833. If now the prism be divided by a plane section oblique to the axis, the two rays co-incident in direction, as above supposed, will be unequally reflected by this plane. And the ordinary ray will suffer total reflection at an angle at which the extraordinary ray is almost totally transmitted. The angle of total reflection for this ordinary ray is 37° 11', while that at which total transmission occurs for the extraordinary ray is 34° 2'. From 34° to 37°, the amount of light reflected from the extraordinary ray is inconsiderable; amounting at the latter angle not quite to eight one-thousandths of the entire ray, and to four one-thousandths of the total intensity of the ray originally incident upon the prism. If, therefore, the supposed calc spar prism were cut by a plane, making an angle of 37° 11′ with one of its lateral faces, a ray incident perpendicularly upon this lateral face and meeting the plane of section, would be half reflected and half transmitted,