croscopes, many of which were the long-tubed, complicated models. Very soon, however, they would become disgusted with their own property, and discard the large instrument for the one furnished by the College. The objectives required are only two, I think, at least for the beginner; an one-inch, and an onequarter or one-fifth inch. Only two oculars are necessary-one with a micrometer-scale. The higher powers are generally employed for the study of amoeboid movements, or some special subject, and a collection of half a dozen immersion one-tenths, with perhaps one or two even higher, will suffice for twenty-four students. In like manner, two or three polariscopes and spectrum-oculars will be all that is needed by two dozen men. Perhaps each man should have a microtome, though free-hand cutting is often quicker and better. The freezing microtome, I am confident, has but a limited use, provided the tissue be properly hardened. There are many objections to the promiscuous use of the freezing microtome. My own experience then, taken for what it is worth-no less, no more— convinces me that a simple microscope, and a comparatively few accessories, combined with a desire to learn, and plenty of material, produces the best results in a student's laboratory.

UNIVERSITY OF NEW YORK.

Photo-micrography.

BY PROF. C. HENRY KAIN.

Photo-micrography, or the art of photographing large views of microscopic objects, is no new thing, but I doubt whether microscopists in general are fully aware of the extent to which the late improvements in dryplate photography have simplified the work.

To the investigating microscopist, it is almost absolutely essential to be able to permanently preserve the re

sults of his observations. This is usually done by the aid of the camera lucida, and the zealous worker will often sit for hours with his eye fixed at the instrument, laboriously striving to represent an object, and, if he is not well-skilled in the use of the pencil, his labor is frequently almost useless, so inaccurate is the result. By far the greater part of this kind of labor may be saved at an expense so trifling, and with results so satisfactory, that I think the time is at hand when every working microscopist will regard a dry-plate photographic outfit as a necessary part of his equipment.

The wet-plate process is cumbersome and not well adapted to the wants of the microscopist; but the dry-plates now in the market are admirable, not only for their great sensitiveness and beautiful results, but also for the ease with which they can be manipulated. They can be purchased so cheaply that it will scarcely pay the microscopist to prepare them himself. Some of the great advantages which they possess are the following:

1. They can be kept for any length of time and used as occasion requires.

2. If not convenient to develop the plate at the time the exposure is made, it can be put away and developed at leisure, even after an interval of weeks.

3. No dangerously poisonous chemicals are necessary in the developing process.

4. They are so sensitive that the light of an ordinary kerosene lamp (preferably a student lamp), is amply sufficient to photograph objects with all powers not higher than a half-inch objective. Indeed, I think it probable that a quarter-inch objective could be so used by properly arranging a system of condensers.

The apparatus which I use is a small camera about eight inches square, such as is furnished with the amateur photographic outfits now so popular. To use it for photo-micrography I simply substitute my micro

scope for the ordinary camera-lens. In order to make a perfectly lighttight connection between the camera and the microscope, I fasten over the opening of the camera a sheet of thick rubber having a hole cut in it somewhat smaller than the body of the microscope. Then, when the body of the microscope is thrust through the opening, the rubber closes tightly around it and makes a perfect joining. The eyepiece can then be inserted from the inside of the camera, if desirable. In photographing without the eye-piece there is apt to be a bright spot in the centre of the field of view, due to irradiation from the interior of the body the microscope. The use of the eyepiece entirely obviates this difficulty, and, at the same time secures greater magnification; but, of course, the field is not so brilliantly lighted. If, however, it is desirable to dispense with the eye-piece, the bright spot can can be avoided by lining the interior of the tube with paper which has a dead-black surface.

As it is desirable that the object shall be as well illuminated as possible, it is best to take the light direct instead of reflecting it upon the object by means of the mirror. It is also best to use two condensers, a large one for collecting as many rays as possible and concentrating them upon a smaller condenser, which, in turn, concentrates them upon the object. An achromatic condenser may be advantageously substituted for the latter.

As some who desire to experiment in this line may desire a starting point as regards the matter of exposure, I would say that with the light of a student lamp, and using a single condenser, I have found that from 11⁄2 to 22 minutes with a 2-inch, 22 to 5 with a 1-inch, and 4 to 7 minutes with a 1⁄2-inch objective, are about the proper times when the A eye-piece is in, and using what are known as Carbutt's rapid (B) plates, No. 468. When the eye-piece is not used about one-half of that time is required. Of

course the time of exposure will vary somewhat, according to the density or transparency of the object, and, if stained, according to the character of the coloring matter.

Apparent Motions of Objects.

BY PROF. F. C. VAN DYCK.

Objects viewed through the microscope seem to move when the position of the mirror is slightly changed.

This fact is doubtless familiar to most microscopists, but has not, so far as I know, been discussed in its optical bearings.

The phenomenon is easily observed by using nearly parallel rays to illuminate the object, and placing the mirror approximately central under the stage. If daylight is used, set the microscope at a considerable distance from the window, and use the planemirror. If lamplight is used, set the lamp at the focus of the concave mirror, or use a lens to make the rays parallel and reflect them from the plane-mirror.

If the object be so thin as to be sensibly in one plane, it will maintain its location in the field whatever change be made in the position of the mirror, so long as it is accurately focussed. But if the tube of the microscope be raised or lowered, so as to throw the object slightly out of focus, a shifting of the mirror on its bearings will cause an apparent motion of the object to one side or the other.

If an object of considerable thickness be used and the focus obtained for a central plane, rocking the mirror will cause the lower parts of the object to move to one side, while the upper parts move to the other side. I have an insect's foot with claws, which, treated in this way, seems to work the claws like scissors. Minute details of an object may be made to disappear under spots on the coverglass, and various similar effects can be produced.

Let us suppose that the illumination is received from the left of the observer, and that a micrometer is inserted in the eye-piece to facilitate observation. Take three points, A, B and C, in the optical axis, A, beyond the focus of the objective, B at the focus, and C a little above the focal plane. Suppose a pencil sent from the mirror along the axis, passing A, B and C, and the centre of the objective. The images of A, B and C will fall with their centres on the axis. If the edge of the mirror toward the observer's right be tilted up, the point A, beyond the focus, will appear to be displaced toward the right of the field of view, the point B will remain stationary, and C, which is above the plane focussed upon, will move toward the left. Now it can be shown that if the spherical aberration of an objective could be corrected for a series of points and their images, all the images must remain stationary.

The necessity for correction consists essentially in the fact that the margins of lenses with spherical surfaces are too strong relatively to their centres. Hence, with an uncorrected lens, the image of the point B, made by the central portion of the lens, would fall on the axis; but an image of the same point, produced by rays entering the left-hand margin, would fall to the left of the axis, as well as nearer to the lens. The essence of correction is to relatively weaken the action of the margin, so that the image shall fall on the axis, and at the same distance from the lens as the image formed by its central portion.

Suppose this correction to be made for the point B. Let the mirror be tilted as described, so that a pencil of rays passes through A to the left margin of the objective. This pencil makes a smaller angle with the front surface than a pencil coming from B and entering at the same place. Hence, the pencil from A will reach the back surface of the lens at a point nearer the axis than would be reached

of 60° C, and leave it for the same length of time. Indeed, if care be taken that the temperature does not materially exceed 60°, the specimen may remain as long as convenient. When the tissue is thoroughly saturated with melted paraffine, a small paper box may be filled with melted paraffine and the specimen placed in it to cool. If properly imbedded, a cut surface has a smooth and shining appearance. No line of division must appear between the specimen and surrounding paraffine. The whole mass should cut, as nearly as possible, like one homogeneous mass of paraffine.

The subsequent handling of the sections varies with their nature. Moderately thick sections of firm tissue may be placed in turpentine to remove the paraffine and mounted as usual in chloroform-balsam. Thin specimens, or those which come to pieces when the paraffine is removed, like thin sections of liver, etc., may be laid on the slide on which they are to be mounted and the paraffine washed out by benzine, carefully applied with a dropping-tube; allow the benzine to evaporate, then lay on the cover-glass and apply thin chloroformbalsam at the edge of the cover. For exceedingly delicate specimens, such as embryos or osmic acid nerves, another method may be used. Lay the section on the slide, wet with absolute alcohol and let the alcohol completely evaporate, leaving the specimen attached to the slide; carefully heat until the paraffine is softened, or slightly melted. When cool, let a few drops of benzine-best applied with a brush-run over the section until most of the paraffine is gone. When dry, apply the cover-glass and put a thin solution of Canada-balsam in xylol to its edge. The xylol may be used instead of benzine but it is more expensive.

This method is very convenient, especially for histological laboratories. The specimen once imbedded, can be kept for years, and new sections cut as wanted. No change takes

place in it nor can it dry up. It is suited to all tissues. I have imbedded all vertebrate soft tissues, chick and trout-embryos, hydras, snails, angle worms, clams, star-fishes, etc., with equal success in every case.

The ease with which the sections can be made, fully compensates for the time required to imbed. The merest tyro, provided with a good section-cutter, a brush to keep the sections from rolling, and such a specimen, must be a bungler indeed if he cannot cut at least thirty even sections from each millimetre of a moderate-sized specimen such as the œsophagus of a rabbit. With a little. practice he should be able to cut a millimetre into one hundred sections without losing more than two. The writer has cut a frog's spinal cord so imbedded into 926 sections mm. thick in one day, and mounted them without losing any sections. No one who practises with these specimens will regard this as much of a feat. It is simply a hard day's work.

Specimens as large as the central hemisphere of a rabbit can be stained and imbedded whole.

I append my notes on the spinal cord of a frog, showing the times used in the various processes:

Cord put into 3 per cent. nitric acid, two hours.

Seventy per cent. alcohol, six hours. Stained in hæmatoxylin, four hours. Seventy per cent. alcohol, over night.

Ninety-five per cent. alcohol, twenty-four hours.

Oil of cloves, twenty-four hours (did not wish to imbed till next day); then,

Turpentine stir half-an-hour.

Turpentine and paraffine, one hour.
Paraffine, one hour.

It should be remembered that these cords imbed easily.

One caution further-select paraffine if possible, which is bluish-transparent and which rings slightly when struck. The white, opaque sort is by no means as good. Any addition of

paraffine-oil, turpentine, etc., to soften the paraffine, renders it granular and brittle, and is decidedly injurious in its cutting qualities.

EDITORIAL.

Subscriptions.-Remittances for subscription should be made by post-office money-order, by drafts payable in New York, or in registered letters. Money sent in any other way will be at the sender's risk. A receipt will be immediately given for money received by open mail.

SYNOPSIS OF RHIZOPODS. - This book has been greatly improved by the addition of four lithograph plates, illustrating each genus of fresh-water rhizopods by at least one representative species. The price has, therefore, been raised to $1.00, instead of 75 cents as heretofore. Every species described by Prof. Leidy in his larger work is also concisely described in this book, and with the aid of the plates it is believed that species can be readily determined. Rhizopods are so abundant, that they have only to be looked for to be found, and they afford an inexhaustible source of pleasure to one who studies them

with care.

MICROSCOPICAL LABORATORIES.We are pleased to be able to publish this month a contribution from Dr. J. W. S. Arnold upon this subject. That it is from an experienced teacher, no one can doubt who will read the article carefully. We have nothing to add to what he has written, but we can say that the ideas expressed in that contribution are precisely those which we can most heartily commend to the attention of every teacher. Probably a wholly erroneous notion of the cost of fitting up a suitable microscopical laboratory has deterred many of our smaller medical colleges from teaching histology with the aid of microscopes; and it is a disgrace to the profession that so many hundreds of "full-fledged" physicians graduated every year without having looked through a microscope.

are

Microscope-stands for laboratory use can be bought by colleges for about twenty or twenty-five dollars each, and the two necessary objectives, good enough ones too, need not cost over that amount, so that from forty to fifty dollars is enough to buy such microscopes as are required.

PHOTO-MICROGRAPHY.-The article we publish this month upon this subject, calls the attention of microscopists to the dry-plate process which is sure to give a great impetus to the photographic delineation of microscopic objects. The process is cleanly, and eminently well-suited to the wants of microscopists who cannot use the pencil well. Next month we intend to give another article on the subject, in which full instructions for using the dry-plate process will be found. It now remains for some enterprising manufacturer of photographic goods to introduce an apparatus for microscopical use, at a reasonable cost.

THE MICROSCOPE IN MEDICINE.Perhaps it is not well to speak the truth too plainly, but sometimes it has a salutary effect. We have just alluded to the lamentable ignorance concerning the microscope among the graduates of medical colleges. If any argument were needed to convince the general reader of the want of interest in practical microcopy among physicians, we would only state that as a matter of business, we have found that it does not pay to send sample copies of this JOURNAL to the physicians in regular standing in State or County medical societies. On the other hand, it does pay to send them to the addresses given in general directories. We do not know how many physicians there are in the country, but if we had on our subscription-list even five per cent. of the total number, we venture to say our present list would be doubled. True, this is not a medical publication, but if the medical profession, as a whole,