often impossible to distinguish as to the relative ages of the two. But specimens in my collection offer conclusive proof that each of the following cases occur.

I-The copper was present before the calcite began to form, and became enclosed in the growing crystal.

In this case the copper and its associated minerals generally form the basis on which the calcite rests, and the crystals of the latter exhibit entering faces wherever the surface of the crystal is in contact with the copper; it should seem to indicate an effort at those points to crystallize free from the foreign substance, by forming separate individuals. But on the finished crystal the traces of this tendency is visible, generally, only in the comparatively very small entering faces at the contact with the copper.

In this way calcite crystals, formed in a cavernous mass of copper, are intersected internally by a perfect net-work of thin plates of the metal, and yet preserve their cleavage unaffected; but wherever the copper comes in contact with the surface of the crystal, the small entering faces are present.

II.—The crystal of calcite was partly formed, then became incrusted with copper, and was finished by a new growth of calcite over the metallic film.

A most remarkable instance of this case is that of a crystal about 2 inches long-a steep scalenohedron-with a basal termination of about 1 square inch surface. At this stage of its growth it was covered, over nearly the whole surface, with a thin coating of copper. The basal termination on scalenohedrons of calcite is as rare on Lake Superior as elsewhere, and in the few instances where I have seen it, it lacks the polish which indicates perfect growth. The tendency to complete the point of the scalenohedron is well shown on this specimen; over the partially copper-coated basal plane there are scattered a large number of perfectly pointed scalenohedrons—two or three of these are to inch high-and others are scattered over the side-faces. All of these younger crystals are arranged in perfect uniformity with the plan of the underlying, older individual.

Those portions of the surface on which the copper-coating is perfect have no younger calcite crystals; these occur where the metallic film is thinnest and more or less perforated.

The copper is not confined absolutely to the surface of the crystal on which it lies; it penetrates to a slight distance along the cleavage-planes, and the result is an exceedingly delicate treiculation on its under surface. The calcites which are planted on the copper contain brilliant particles of the me tals

swimming, if one may use the word, in the interior of the crystals; and these are so disposed as to lead to the idea that throughout the growth of the younger crystals they had to contend with the continued deposition of the metal. Thus one of the new scalenohedrons, after growing to the height of inch, was, like the underlying one, also ended with a basal termination, on which again smaller new and well pointed individuals were built up.

III.--The copper has entered the calcite crystal since its growth was finished.

A specimen, in my collection, illustrates this remarkably well. It is a cleavage-rhombohedron of opaque calcite, traversed by intersecting sheets of copper, which are wholly independent of the cleavage planes. On detaching the copper from the calcite, the surface of the latter appears rough; it is a fracture oblique to the cleavage, and the face of the fracture is formed by countless corners, or solid angles, of minute cleavage-rhombohedrons, as is fully proved by the reflexion of the light. The coppersheets, which are about inch thick, reproduce this very com

pletely.

Another very remarkable specimen is from the cement of the Albany and Boston conglomerate. It is about 1 inch in diameter, and consists of opaque white calcite. The continuity of the cleavage shows it to be a single individual, though it passes on the edges without any sharp demarkation into the common cement of the conglomerate. This calcite is traversed by continuous sheets of copper to inch thick, which are perfectly straight. These sheets are parallel to several planes, (nearly all of which are independent of the cleavage) and intersect each other. In each of the sets thus formed the sheets are perfectly parallel, and are separated by plates of calcite, which are in places as thin as the copper itself. Where three such sets intersect each other, the resulting solid appears composed of concentrically arranged laminæ of copper and calcite. In some parts of the specimen the copper predominated over the calcite. Wherever the faces of the copper laminæ are exposed, they are marked with a delicate, reticulated tracery, indicating the lines of intersection of the sheet with the cleavage planes of the crystal. The cement in the vicinity of the calcite is impregnated with copper; in places it is almost wholly replaced by the metal in the fine granular condition called "brick copper," and into this the laminæ of metal extend, without break, from the calcite. This specimen is really a pseudomorph of copper after calcite.

Copper and Silver.-It is a well known fact that these two metals occur in the metallic state, in the Lake Superior depos

its, in the most intimate contact with each other, and yet without being mutually alloyed. Even at the contact they are not absolutely joined together, for after rolling out a piece of copper containing spots of silver, the two metals become more or less separated, and may often be readily detached from each other. I have not been able to obtain any material that would throw light upon the relative ages of the two metals. [To be continued.]

ART. XXXIV.-On Photographing Histological Preparations by Sunlight; by J. J. WOODWARD, Asst. Surgeon U. S. Army. Report to Surgeon General J. K. Barnes, U. S. Army. Washington, June 9, 1871.

IN January, 1870, I had the honor to submit to you a report in which I detailed the results of a series of experiments, which showed the superiority of the electric and magnesium lights over sunlight, as heretofore employed, for the production of photo-micrographs of the soft tissues. In June of the same year I made a report in which I showed that similar results could be obtained with the oxy-calcium light. With these various artificial sources of light, I obtained pictures which appeared to me to be "clearer and better defined than any photographs of similar objects I had hitherto seen produced by sunlight.'

So many cloudless days are offered to the photographer in Washington, that I could not but regret these results; yet they appeared to be final at the time of writing. During the last few months, however, I have found improved methods of using the light of the sun for photographing the soft tissues, and have arrived at results which must materially modify the conclusions of my former reports.

Not that I have anything to withdraw from the opinions I have expressed, as to the certainty and success attending the use of artificial lights for the purpose named, but I have much to add with regard to the most advantageous methods of using the light of the sun for obtaining satisfactory pictures of tissue preparations, and such other objects as approximate them in optical characteristics.

If a well made preparation of some normal tissue, or of some pathological growth, stained with carmine, silver, or gold, and mounted temporarily in glycerine, or permanently in Canada balsam, be illuminated by white cloud illumination, or by lamp light, and found to be all that could be desired, it will nevertheless appear very unsatisfactory if illuminated by the direct rays of the sun.

The eye glancing through the tube of the instrument, dazzled by the powerful light, discerns amidst the blaze, innumerable colored rings, produced by diffraction and interference which disturb the normal appearances of the preparation and render its interpretation impossible.

If the image be received upon a white screen similar phenomena obtrude themselves, destroying the clearness of the picture, though no longer injuriously affecting the eye; and if monochromatic light is employed, although the disorderly play of color disappears, black rings and lines of the most manifold character and direction take their place. Pictures produced under these circumstances are of course quite useless, and the difficulty occurs not merely in the case of tissue preparations, but in a very large number of other objects.

To escape these disagreeable results, it has heretofore been the practice to pass the solar pencil through a piece of ground glass. This plan is recommended in all the treatises on photo-micrography, and has hitherto been employed in the solar work done at the Army Medical Museum. The method is effectual in getting rid of the diffraction and interference phenomena complained of; an image is obtained which is clear and satisfactory to the eye looking down the tube, but it appears very weak on the screen, and is sadly deficient in contrast. These faults are reproduced in photographs of objects thus illuminated, and, moreover, the time of exposure is enormously increased. Such pictures are decidedly inferior to those which can be obtained by the magnesium, or even by the calcium light, with which no ground glass is used.

I desire now to call your attention to the fact that in the course of some recent experiments, I have ascertained that the diffraction and interference phenomena above complained of, may be prevented by the use of a suitable condensing lens, even better than by the ground glass; that by this plan the exposure may be greatly diminished, say from three minutes for five hundred diameters, to a fraction of a second, and that the resulting pictures are not merely quite as free from diffrac tion and interference phenomena as the best that can be obtained when the ground glass is used, but are characterized by greater contrast and superior sharpness of definition.

The details of my new method are as follow: The microscope being placed on a shelf at the window of the dark room, and its body made horizontal, the achromatic condenser is illuminated by a solar pencil reflected from a heliostat upon a moveable mirror outside the shutter and thence into the dark room, precisely as described in my original paper on photo-micrography.*

* This Journ., II, vol. xlii, p. 189, Sept., 1866.

No ground glass is used, but instead a lens mounted in a suitable tube is fixed in the opening of the shutter through which the solar pencil enters. This lens is an achromatic combination about two inches in transverse diameter and of about ten inches focal length. It is placed at such a distance from the achromatic condenser that the solar rays are brought to a focus and begin again to diverge before they reach the lowest glass of the achromatic condenser.

For anatomical preparations requiring for their display from two to five hundred diameters, I use an th of an inch objective, without an eyepiece, obtaining the precise power desired by variations in the distance of the sensitive plate from the stage of the instrument. I have lately given the preference to immersion objectives, the corrections of which I find are generally well suited to photographic requirements..

Now with an 1-8th objective and the arrangement above described, the field is so brilliantly illuminated that the eye cannot safely be permitted to look down the tube. The image is therefore received on a piece of white card-board, and sitting by the microscope to make the adjustment, I view the card with both eyes precisely as in the case of the ordinary solar microscope. With these arrangements, the card-board placed from two to four feet from the stage of the microscope is sufficiently well illuminated to permit distinct vision, even when objectives of the shortest focus are used and powers of five to ten thousand diameters obtained. While the object is thus seen on the white screen in its natural colors, the cover corrections, focussing, management of the achromatic condenser, and selection of the portion of the preparation to be photographed, are readily managed. When all is satisfactory, I insert an ammonio-sulphate cell between the large lens and the achromatic condenser, and draw down the velvet hood which prevents leakage of light from about the microscope into the dark room; then going to the plate holder, I make the final focussing in the usual way on the ground glass, or on plate glass with the help of a focussing glass, according to the nature of the object.

With powers of five hundred diameters or less, I at first experienced some difficulty in giving the right exposure; for as the time required was but a fraction of a second, it was a matter of some difficulty to regulate it with precision. At length I succeeded by arranging a sliding shutter, with a transverse slit of variable width, so adjusted as to fall with its own weight before the tube of the microscope, the exposure being made during the passage, and the time of exposure regulated by the width given to the slit.

Of course it occurred to me that for such short exposures the heliostat might be dispensed with, and I found on trial without