all ranged in one line. By means of the brass furrow in the base board the distances between them were changed without getting out of line. The sunlight, the chemicals, and all else had previously been found in working order by practical tests. Sunlight was thrown by the mirror through the condenser on the object which was placed just beyond the heat focus. We found that the brightest and clearest days, before 3 P. M., were the best. One observer, with his head and the camera covered with a black cloth, noted the projection of the image on the glass-ground plate. Another fingered the fine adjustment, or it was done by the focussing rod. When the image was satisfactory a card board cut off the light by interposition between the condenser and the object. The sensitized plate then replaced the glass plate and exposed, the regular exposure was made by lifting the card board and letting it fall in the course of half a second or more. The time varies and must be learned by trial. Usually it is shown by the action of the chemicals on the exposed sensitive film in the dark room. The processes afterward were those of the ordinary collodion process. It was necessary of course to look over the printing and instruct the printer how much exposure was needed.

In photographing yeast with the inch objective the object was wet and covered with a film of mica. The following facts may not be out of place. It was made by Robert B. Tolles, at Boston, and delivered July 1st, 1873. It was ordered by Dr. Harriman for the sake of working up his demonstration of the presence of nerve fiber in dentine. (American Journal Dental Science, May, 1870, Dental Cosmos, Jan., 1870.) Its angular aperture is 170 degrees; its actual opening on the face, inch. Cover adjustment moves about circle. Works wet or dry. Requires the aid of a powerful condenser. Usually it works best with a B eye-piece as a condenser under the stage, and with the thin edge of a common coal-oil flame shining "direct" into the condenser. It has to be used on a first class stand whose stage is absolutely at a right angle to the tube. With this direct light the field is clear, white and flat. The objective is very sensitive to jars and motions. This troubled us much. We have found in our experience that a cellar in a locality away from avenues of travel is the best place to work in. When an object is in focus with this objective a gentle grasp of the arm that connects the tube with the trunion joint (see cut) is sufficient to move the object out of focus.

The comparative excellence of this objective is not one for much discussion here; some have hastily said that it was of no value, not having used it, while others have looked at it with a sort of awe. In our opinion the question is not settled, though we think something toward it has been done. As far as

AM. JOUR. SCI.-THIRD SERIES, VOL. XVIII. -No. 104, August, 1879,

our work has been concerned we know that we could not have attained our results with another objective like the for instance, with the ease and facility with which we did with the While we feel sure that the practical clinical results of corroborating our study of consumptive blood can be attained with objectives of inch power-and it would be sad if it were not soat the same time we are sure that no wrong has been done to any one by pressing the into our service. Moreover, if by our simple arrangement we have been able to transfer images with the highest power objective ever thus used, those who possess the low powers ought to be encouraged to use microphotography with the sunlight without condensing, or with the ordinary mirror, or with the B eye-piece.

Figure 2 is a section of the writer's device for such work; it may be gotten up at a trifling expense. a is the tube of the microscope; b is a paper tube 30 by 2 inches. A nicely turned plug of wood adapts the microscope to the paper tube. To save space, the tube is broken off in the cut; a deal 8 by 12 by inches is seen in section, and fitted by a hole to the paper tube b. c is a section of the ground glass plate and holder, d is the clip to hold the plate holders. The artist has omitted the section of the lower cleat. This apparatus is adapted to a quarter plate and a two-inch photograph. An assistant should focus and adjust the light.

With these simple arrangements it would seem that the hope expressed at the outset of this article should begin to be realized. Tremont Temple, Boston, April, 1879.

Postscript. The first microphotograph of this objective may be found in the Yale College Library.

ART. XVI.-Magnetic Strains in Iron; by A. S. KIMBALL, Professor of Physics in the Worcester Free Institute of Industrial Science.

THE object of this paper is to describe certain experiments made by inducing a magnetic state in bars of soft iron subjected to varying degrees of mechanical stress. As the result, we always have changes either in the form or dimensions of the bar, similar to those produced by the mechanical stress previously applied, and therefore the term magnetic strain does not seem inappropriate. Some of the phenomena hereafter to be described have been observed by earlier investigators. These experiments have not been repeated with the expectation of detecting errors in their work, nor of attaining a higher degree of accuracy, but rather, to afford that valuable check which the reproduction of well settled phenomena, with a new disposition of apparatus, affords, both upon the accuracy of the instrument and the skill of the operator.

Effect of Magnetization upon the Tenacity of Iron.-The pieces of iron tested were pulled asunder by a Fairbanks testing machine of 53,000 pounds capacity. The machine consists: 1st, of a large platform scale; 2d, of a powerful screw-straining apparatus, driven by a belt from a shaft having eight changes of speed; the motive power is a Corliss engine which runs with great regularity; 3d, an automatic weighing attachment to the beam, by which it is kept constantly poised as the stress is applied to the test piece.

The delicacy of this adjustment was such, that when the testing proceeded at a suitable rate, the deflection of the beam from the zero point did not indicate a stress on the test piece differing more than two pounds from that shown by the posi tion of the poise on the beam. The scale was "sensitive" to the addition of one ounce when the platform was loaded with 4,000 pounds; and on the removal of the small weight, the beam promptly returned to its normal position. The course of experiment was as follows: Several pieces of the same kind of iron, made as nearly as possible uniform in size, were broken in the machine. The alternate ones, in the order in which they were cut from the bar, were magnetized to saturation by a helix, through which a constant current was passing during the experiment. The heating effects of the helix were slight, and probably without influence. The tabulated results were then compared, and from thein the following conclusion was reached: A soft iron bar has its tenacity increased about nine-tenths of one per cent by magnetizing it to saturation. The following table gives the results obtained by breaking a series of pieces of

annealed iron wire, very uniformly drawn; approximate diameter, 1623". Length between the jaws of the machine, 5". Time required to break the magnetized pieces, sensibly constant at five and one-quarter minutes; for the unmagnetized pieces slightly less.

Another series of experiments upon telegraph wire gave 8.9 lbs. difference between the means. Seventeen pieces of wire were broken. A series of ten wires, one-quarter inch in diameter, gave an average of unmagnetized pieces, 4532 lbs., of magnetized specimens, 4572 lbs. Difference, 40 lbs.

Some hundreds of pieces were broken with the same results. A magnetized piece always proving stronger than the unmagnetized pieces taken from the coil or bar on either side of it. A few apparent exceptions to this rule showed flaws in the tested pieces on close examination. The average increase of strength in these experiments is very near nine-tenths of one per cent. In every case the strength of the unmagnetized pieces was much more uniform than that of the magnetized. In the Philosophical Transactions of the Royal Society, 1874, p. 571, Sir William Thomson predicts this result as a deduction from Mr. Gore's experiments on Electro Torsion.

Effect of Magnetization upon the Flexure of a soft iron Bar.— Joule's experiments upon the changes in dimensions of an iron bar when magnetized, formed the starting point for this part of the experimental examination in question. He has shown,* 1st. That if an iron rod be compressed longitudinally, it will be slightly elongated upon being made a magnet by a surrounding helix. 2d. That the amount of compression does not affect the magnetic extension so long as the magnetizing force remains the same. 3d. That the same phenomena, in kind and

* Philosophical Magazine, 1847.

amount, occur in a bar which is neither compressed nor stretched. 4th. If the bar be subjected to tension, the elongation, on making it a temporary magnet, is diminished, and as the tensile stress increases the magnetic elongation diminishes through zero and becomes a shortening. 5th. Professor Mayer has shown* that, in the case of an unstrained bar, the first passage of the current elongates the bar more than any subsequent passage of the same current, and that the second, third and all subsequent elongations of the bar by a constant magnetizing force, are equal to each other; also that the shortening of the bar upon breaking the current is constant and equal to the second elongation. These facts, taken in connection with the common theory of flexure, fairly indicate one or two phenomena which will be found to attend the induction of a temporary magnetic state in a bar strained transversely. We see, from what has been said, that the neutral axis, and all the fibers on the concave side of the bar which have been shortened by compression, will be elongated by the action of the magnetizing force, while the fibers of the bar on the convex side, which have been subjected to tensional strain, will either be elongated by a less amount or will be shortened. As the result of this action we may be tolerably sure that the bar will be straightened. It is much safer, however, in this case, to proceed with our investigation experimentally, since neither the theory of magnetic action in iron, nor that of transverse elasticity, can be said to have been fully developed.

The apparatus used in this part of the investigation was simple. A very rigid iron casting, with supports for a micrometer screw, and the ends of the iron rod to be examined upon its upper side, was placed upon the platform of a Fairbanks scale. The iron rod, carefully freed from magnetism and enclosed in its helix, was adjusted upon its supports so that the point of the micrometer screw was just below its middle. The helix was made in two parts for convenience in loading the bar. The middle of the bar supported one corner of a triangular platform, whose sides were four, eight and nine feet. The other corners of this platform were supported upon points. This disposition of apparatus proved very satisfactory. The load upon the bar was easily and accurately determined by the scales, while the stability of the triangular platform permitted the addition or removal of weight without seriously disturbing the adjustment of the bar. The micrometer screw had sixty threads to the inch, and its head was graduated to three hundred parts. The unit of measurement is therefore 18 of an inch. At first, contacts of the screw with the bar was deter

*This Journal, August, 1873.