The student frequently finds in the course of his observation upon living and other tissues, features that are vital towards proving the truth of his researches, but so evanescent, that the lapse of a few minutes may suffice to obliterate them. A small simple camera which can be at once applied to the microscope, without the slightest alteration of the latter, save inclining the body to a horizontal position, using the same source of illumination, be it diffused daylight or that of the ordinary lamp, would be a boon, Fig. 97 shows such a camera.

The camera consists of a mahogany box about 2 3/4 in. square,corrugated and blackened on the inside to prevent any reflections of light, A solid cone some 4 in. long, tapering to receive the tube of the microscope, is attached to the front of the box. Preferably, this cone-front should be in bellows form, an in the figure shown; but being rather more costly than the solid cone, many will be satisfied with the latter. In the one case the bellows responds readily to the movements of the microscope tube in focusing; in the other the tube .must slide readily into and ont of the solid cone. At the opposite end of the box is a groove, in which the plate-bolder and frame containing the focusing screen slide. The former carries two plates 2 1/2 in. square, amply large for alt ordinary illustrations. Should larger sized pictures be required, they can he made by enlarging upon bromide paper. The focusing-screen is made of very thin crystal glass, most carefully ground by hand, presenting the smoothest surface obtainable by this means, but still quite too coarse for the exact focusing of delicately marked objects. In fact, the focusing-screen is mainly useful in procuring even and full illumination of the field, and in properly centring the object.

The final filing of the exact focus is done by means of a focusing-glass used in conjunction with a disc of thin cover-glass attached to the ground surface of the screen by means of Canada balsam.

Fig. 97.

Photo Micrographic Camera 10080

The camera is mounted upon a stout metal rod, which slides into the upright shaft of a very heavy japanned base, and can be secured at any height to suit that of the microscope (when the latter is inclined to a horizontal position) by means of a milled head. The base is shod with thick felt cloth, so that it may be placed upon any polished table-top without scratching the latter, and at the same time remain firmly fixed in the position it may be placed in.

And this is all there is of it: Simple, compact, always ready for immediate service, and occupying no appreciable space upon the work-table. Although primarily intended for use with the microscope body inclined to a horizontal position, it may be as readily adapted to the latter in a vertical one, when the character of the objects (as those mounted in fluids) may require. Remove the camera from its base and mount it upon the top of an open box containing the microscope. An opening in the top of the box allows the cone to be slipped over the tube of the microscope.

Illumination may be effected by reflection from the mirror as in ordinary work, or by removing the latter and placing the lamp behind the stage, and in a direct line with the optic axis. It must be carefully centred in order to illuminate the field alike in all portions. Condensers of various kinds - bull's eye, achromatic, Abbe, etc. - can be used as desired, but with moderate and low powers. The best results will be obtained by the employment of simple diaphragms of various sizes to suit, and so placed as to come as close as possible to the under surface of the slide- upon which the object is mounted. All extraneous light should be excluded so far as possible, and none be allowed to enter the objective other than the rays which illuminate the specimen. Opaque objects may be photographed quite as successfully as transparent ones; but the time of exposure will be very greatly shortened by employing direct sunlight as the illuminant, if possible.

The eyepiece may be removed or not, as the observer may elect. The late Dr. J. J. Woodward almost invariably worked without it, using an amplifier where sufficient magnification could not be obtained with the objective alone. In using medium and high powers, the eyepiece is not objectionable; but with low powers, it certainly detracts from sharpness of definition, so that the amplifier is better where an increase of power beyond that obtainable with the unaided objective becomes necessary. If possible, however, always use the latter alone. The short tube-length, alone possible, renders the employment of amplifier or ocular necessary, if enlargements beyond 300-400 diameters are to be made, since the limit of a 1/18 used direct is less than 350°.

The corrections of most modern objectives as to visual and actinic foci are so nearly identical, that no difficulty will be experienced in obtaining sharp definition of any subject if a little care be used.

The dry plates are furnished by the makers in two degrees of sensitiveness to suit every variety of subject. They are readily developed by any of the methods used for gelatine plates, pre-ference being given to hydroquinon, or a mixture of that with eiokonogen, as giving the clearest results, clearest details, and sharpest contrasts with any desired amount of density. Their post is but 25 cents per dozen: certainly cheap enough to tempt any. one to their use.

Bromide paper is the best material for printing in this class of work. Using the smooth surface-paper, and developing with ferrous oxalate, we get a perfect print, rendering the most delicate details, with the crispness and clearness of a steel-plate engraving, which indeed it most closely resembles in very many instances. The exposure is made by lamplight, so that one is entirely independent of time and weather, and the finished print is absolutely permanent; as much so, it is reasonable to believe, as a carbon print. If the sheet be allowed to dry spontaneously, it will present the appearance of an ordinary plate engraving. If a polished surface be desired, all that is necessary will be to float the paper, print side down, upon a sheet of polished hard rubber; to squeeze it into optical contact, removing all superfluous moisture, and when quite dry it will peel off the rubber-plate with a beautiful polished surface, greatly increasing the delicacy of detail in many subjects, especially diatoms.

The ferro-prussiate, or more commonly named "blue-print." method of printing has decided advantages and merits for the work we are considering. It is cheap, as the paper may be purchased ready sensitised, at very trifling cost, and it requires no skill or experience in the using. It is merely necessary to expose to bright sunlight until sufficiently printed (a few experiments will determine this), and then to wash in several changes of water; the result being a bright, permanent blue print upon a clear, white ground, with excellent detail, excepting in the most delicate structures.

The negatives made with the camera are of a convenient size for printing lantern slides by contact. A print on glass is certainly the most perfect of any that possibly can be made, and the importance of this method of demonstration has long since been conceded. Gelatine plates coated on thin glass with special slow emulsions, are furnished by several makers, and any microscopist can readily make his own lantern-slides with a little expenditure of time and patience. - (W. H. Walmsley.)