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When the air in this interspace is replaced by water, the angle becomes 100°, or a little more.

In this experiment the slide and cover are thrown out as of no importance to the solution of the question, viz. of the actual angular dimension of the pencil traversing the object, and transmissible by the objective.

It seems incontestable at all events that more than 82 of angular pencil can traverse the balsam-mounted object, and be transmitted by the immersion objective to the eye of the observer.

Incidentally to this proposition, the following is given when the object is actually in situ and well defined.

Thus, instead of the water in the above experiment, human blood was introduced between the plano-convex lens A, and the front surface of the objective.

As this necessitated, in order to bring the blood disks into view, separating the systems of the objective (by means of the cover adjustment) considerably, the apparent angle of this 170° objective, i. e. the angle taken in the ordinary way, proved to be only 128°. But the extreme ray transmitted when the blood was compressed by the plano-convex lens upon the front surface of the objective, proved to be less (a little) than 100°.

This form, just detailed, of the immersion objective is a “clinical” method, a year or two in use here, and wherein the front surface of the objective becomes the stage of the microscope, a glass "cover," or a lens as above, being applied to thin out the substance viewed, be it blood, urine, or other material fit to be thus put under view.

A natural sequence of all this is the application of such a planoconvex lens at the lower surface of the object slide, the primary object of it being to avoid the excessive reflexion that takes place at the immergent surface of an object-slide in all cases as now used.

Of course, in such an objective as used in these experiments, 170° upwards, the pencil incident upon the immergent surface of the slide must be to reach the full angle of the objective, very nearly parallel with the face of the slide. Immense reflexion is inevitable.

The application of the plano-convex lens to the under immergent surface of the object-slide allows the extreme incident pencil to enter at a perpendicular incidence very nearly. To be sure the convexity of the plano-convex lens has influence to modify this. But by placing upon the plano-convex lens a plano-concave facet lens (Fig. B), the incident rays meet a plane surface and pass on to the object without suffraction.

The increase of light in this latter case is necessarily large, and the influence of that increase upon the result, i. e. the appearance and demonstration of the object, is remarkable.

The convenience of using an incidence of only about 50° to 60° on each side of the axis, instead of nearly 90°, is evident enough. That this plano-convex lens fixed in the centre of the stage, perhaps preferably made achromatic, will be utilized as a condenser, there seems no doubt. In my own hands it seems to doubly demonstrate difficult tests.

Certainly the use of immersion condensers is abundantly indicated in the above simple experiments.

Boston, Mass., U.S.A., May 24th, 1871.

DR. HENRY LAWSON,
Editor.

Boston, May 25th, 1871. DEAR SIR,-I yesterday mailed to your address a paper by Mr. Tolles on immersion objectives. I now wish to make one correction in that paper. The angle of the rays entering the objective with this arrangement is 110° instead of 100° as written. This makes the angle nearer to Dr. Pigott's statement, and farther from Mr. Wenham’s. Please make the change when printing the paper, which I hope is in season for the July issue.

Respectfully,

Chas. STODDER.

PROGRESS OF MICROSCOPICAL SCIENCE.

A Giant Gregarine.-—We have just received from M. Van Beneden a copy of his memoir on the development of gregarines, in which the structure of Gregarina gigantea is fully described and figured. The pamphlet has reached us too late for any fuller notice at the present; but we shall dwell upon it more extensively in our next issue. It seems & most valuable addition to the literature of the subject, and it treats very fully upon the development of this very curious group. The author dwells upon Professor Beale's views on development of tissues. The work is published in the 'Bulletins de l'Académie royale de Belgique, 2me série, tome XXXI, No. 5, 1871.

The Embryos of Calopteryx, Agrion, and Diplax.--One of the finest and most advanced memoirs that we have seen on these subjects is that just published in the · Memoirs of the Peabody Academy, by Mr. A. S. Packard, jun. It is extremely elaborate. After dealing at length with the subjects, the author thus sums up the characters : Since the observations on Diplax were made, and abstracts read at the meeting at Burlington (August, 1867) of the American Association for the Advancement of Science, and published in the 'American Naturalist' for February, 1868, and in the Proceedings of the Boston Society of Natural History' (vol. xi.) for January 22nd, 1868, he has received, through the kindness of Dr. Alexander Brandt, of St. Petersburg, his admirable paper “On the Embryology of Agrion, Calopteryx, and certain Hemiptera.” Brandt's studies were directed chiefly to the development of the embryonal membranes. His conclusions are : "1st. Calopteryx and Agrion are developed according to the type of the development as shown by Metschnikow to exist in the Hemiptera, namely, the germ or primitive band is internal to the yolk. 2nd. In those insects with an internal germ we need to distinguish an embryonal membrane, which is divided into a visceral and a parietal layer. 3rd. The visceral layer (veiled or plaited layer of Metschnikow) does not become united with the extremities, but enters, together with the parietal layer (amnion of Metschnikow), into the formation of the yolk sac. 4th. The formation of the yolk sac, together with the revolution or turning of the embryo on its transverse axis, consists in an independent contraction of the parietal layer of the embryonal membrane." As Mr. Packard's attention was directed to morphological points, he can only infer from the few data given above that Diplax and Perithemis have the same arrangement of the embryonal membranes, and that these membranes later in the life of the embryo form the yolk sac, through the contraction of the parietal layer of the embryonal membrane, as in Agrion, Calopteryx, and certain Hemiptera. As regards the changes of the embryo after the rudiments of the appendages have appeared, they seem in Diplax and Perithemis to be the same as in Calopteryx and Agrion. The embryo of Diplax is much thicker and shorter, corresponding to the shorter, more ovate egg. The attitude of the germ during its turning in the egg is identical with that of Agrion and

Calopteryx. Finally, Brandt's figure 19 may be compared with his figures 8, 8a, and 9, the yolk now being confined to a small area on the back of the embryo, which is now segmented and nearly ready to hatch, the claws being indicated, the eyes formed, the appendages partly jointed, and otherwise much as in the larva.

A New Method of producing Stereoscopic Effect.-In a recent number of Zehender's Monatsblat'there is an account of this new experiment of Listing, who has already done so much in physiological optics. It brings out stereoscopic effect with only one picture, which consists of figures arranged in a peculiar way, and seen with vertical double images. The simplest experiment is to view two lines crossing each

other at an angle of about 30°, with a prism of 4° or A B 59, its base vertical before one eye. No effort must be

made to correct the vertical diplopia. If the prism be put before the left eye, its base upward, the line B B' seems nearer to the eye than A A'. If the prism be turned with its base downward, and before the same eye, the line A A' seems nearer, and B B' more remote. It is found that with the base downward the prism must be weaker

than when turned with the base upward. In gaining B' A' the effect by prisms so weak as these, no double vision

is produced except for horizontal lines—the oblique lines appear to be only two. The same phenomenon may be produced in a common stereoscope by having two similar figures, and pushing one alternately up and down. Two rows of the same letters are arranged on a page like the limbs of the letter X, and viewed as above stated with a vertically deflecting prism; a sudden removal of one now takes place to a considerable depth, while this appearance is at once reversed on turning the prism around 180°. These curious effects can be best produced and understood by means of the diagrams accompanying the article.

The Red Blood-globule.Dr. Richardson, of America, who has lately been inquiring into this subject, publishes some observations in the American Medical Times. He desires to allude briefly to one of the minor points among his observations, which doubtless has been overlooked,-viz. that recorded to the effect that blood crystals of the Menobranchus, when partly dissolved, could be seen to move rapidly, and as if with perfect freedom, in various directions, between the nuclei and external borders of certain corpuscles. This fact appears to his mind much more consistent with the hypothesis of a cell wall enclosing fluid contents than with the doctrine of a homogeneous jelly-like constitution (Beale), or the theory of a crystalloid element "contained in an albuminous framework of paraglobulin" firm enough to preserve the shape of the red disk (Brücke, Stricker); and it seems to him the indication furnished by this circumstance resembles in kind the evidence which sudden dartings of a gold-fish across his vase would be that he was not imbedded in jelly or entangled within a net. Fully recognizing, however, the wisdom of caution against considering any one series of experiments (or, he may add, indeed, any one man's unaided observations, however numerous) as “conclusive proof,” and trusting, therefore, that these researches will lead others to investigate the subject and correct or confirm his results, he concludes his observations.

A Specimen of Diplograpsus pristis with Reproductive Capsules.—Mr. John Hopkinson, F.R.M.S., has recently described a curious graptolite. The chief peculiarity seems to be the presence of reproductive organs. These, which Mr. Hopkinson considers to be representations of the gonothecæ of the recent Sertularian zoophyte, are developed almost immediately opposite each other, from each side of the periderm and throughout its whole length. Though at equal intervals from each other, they are in no even numerical relation to the hydrothecæ, there being ten to the inch. They appear to have budded from the periderm at right angles to the hydrothecæ, and thus have caused the polypary to be unevenly compressed. The most perfect are pearshaped in form, žth of an inch long; and at their narrow end, by which they are attached, about oth of an inch wide. They have apparently been bounded by a single marginal fibre, which is slightly thickened at its edges, and, where the pyrites are removed, has impressed a fine double groove on the surface of the shale. If the fibres were slender tubes, this appearance would naturally be presented; for their outer margins would offer the greatest resistance to compression. The so-called solid axis of the graptolite frequently presents a similar appearance. At the proximal end of the polypary these fibres only are preserved, the oldest or first-formed gonothecæ having fulfilled their function and perished. The distal extremity of even the most perfect is not clearly defined, the impression of the capsule in most cases becoming gradually less perceptible from the proximal to the distal end. Sometimes the capsules are irregularly ruptured, their torn jagged edges being distinctly seen, while one has split along its marginal limit, along the line of the marginal fibre, which appears to have parted abruptly near the distal end of the capsule at one side, and split acutely for some distance along the other side. This would appear to indicate that the capsule may be composed of two membranes joined together at their edges, through which the fibre, if it be not merely a tube formed by a kind of double marginal seam, has run. In no case can a distinct unruptured distal orifice be traced. The gonothecæ present other peculiar appearances. Towards their proximal end they are sometimes longitudinally corrugated or crumpled, or traversed by fibres which extend for some distance into the body of the polypary. Some are much twisted and bent about, occasionally overlapping each other. Between two which thus overlap, or perhaps only come into contact with each other, just at the point of contact and apparently within one of the capsules, are two minute young graptolites, one lying across the other. Each consists of a thin membrane, probably forming the first partially developed pair of hydrothecæ, a minute radicle, and a slender solid axis which is prolonged beyond the membrane. They are similar in form and proportions; but one is a little larger than the other. Its length, from the extreme point of the radicle to the distal end of the axis, is oth of an inch.

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