Wanted, good gatherings of Pleurosigma angulatum, for diatoms, good slides; will give material or cash. Eugene Mauler, Travers, Switzerland. For exchange, for a microscope, a small engine lathe (screw cutting) in nice order, costing $110. Address Dr. J. H. Converse, West Troy, N. Y. Publisher's Department. Angular Aperture of Object-Glasses. In a recent communication to the "English Mechanic," Sections of mauzanita, Calif. laurel, chapparal, Yerba lauta, mountain mahogany, cedar, etc.; cop- Mr. F. H. Wenham says: "120° is perhaps the per and silver ores from Comstock lode, opaque; maximum of angle that has ever been reached in and rare chemicals for polariscope, for any good the microscope object-glass. I have object-glasses mounted objects. F. H. Engels, M. D., Virginia, in my possession which will admit light to near 180°, Nev. but the angle or axial pencil of which, correctly measured, does not reach 100°; yet these glasses easily resolve the most difficult tests known." Wanted, in exchange for a number of fine slides, the polarizing apparatus for a first-class stand. Address F. F. Stanley, 40 Pearl St., Boston, Mass. Gomphonema germinatum, a pure gathering of "Challenger" soundings, Globigerina, etc.; also, Given Away.-A superb pair of 6x8 chromos, 8 European earths. Good marine and guano dia- worthy to frame and adorn any home, and a three toms, unmounted, wanted in exchange. W. M. months subscription to "Leisure Hours," a charmPaterson, Westfield Terrace, Loftus, England. ing 16-page literary paper, full of the choicest Mounted or unmounted crystals of tartaric, ox-stories, poetry, etc., sent free to all sending fifteen alic, red prussiate, chloride and bichromate of potash, for other objects of interest. Address H. B. cents (stamps taken) to pay mailing expenses. Hargreaves, Garnerville, Rockland County, N. Y. The publishers, J. L. Patten & Co., 162 William street, New York, guarantee every one double value of money sent. $1,500 in prizes, and big pay given to agents. Write at once! Slides of black rust on wheat stalk, opaque and transparent; stained fertile frond of Adiantum pedatum, etc., for other mounted objects. Address F. H., Drawer 35, Cherokee, Iowa. Slides of spores of botrychium, and fossil diatoms from Bow, N. H., for other mounted objects. Lists exchanged. Wm. W. Griffin, Lynn, Mass. A Call to the Telephone.-Several devices have been suggested for calling attention to the Slides of cuticle of larva of Macrosila Carolina, telephone, when it is desired to open conversation. showing crystals, in exchange for other objects. Most of them are clumsy, and some require the atE. A. Burdick, Pension Office, Washington, D. C. tachment of a galvanic battery. One of the simplest Palates of mollusca, a great variety, mounted or is described and figured in "Nature." It is the inunmounted, in exchange for mounted slides of in-vention of W. C. Rontgen, and consists principally teresting objects or good material. C. M. Vorce, 164 Lake street, Cleveland, Ohio. Slides of Arachnoidiscus Ehrenbergii, in situ, on seaweed from Santa Cruz, Cal., and Aulacodiscus Kittoni (transparent), for other mounted objects. C. Mason Kinne, No. 422 California street, San Fran cisco, Cal. Mounts of Uric Acid (natural urinary deposits) in exchange for other mounted objects of interest. Address W. J. Weeks, Homoeopathic Hospital College, Cleveland, Ohio. Specimens of the marbles, granites and minerals of Vermont, in exchange for Western minerals, or good fossils; minerals and fossil woods from the far West specially desired. Dr. Hiram A. Cutting, State Geologist, Lunenburgh, Essex county, Vt. For exchange, transparent injections, equal to finest English slides of Thymus and Thyroid glands, stomach, bladder, kidney, mesentary, intestine and tongue of cat. J. T. Joslin, Newburgh, N. Y. Wanted, in exchange for diatoms from northern Wisconsin and Michigan, any well-mounted objects; marine diatoms preferred. W. H. Titus, M. D., Shawano, Wis. of a tuning fork (Ut 4), one of whose prongs is brought close to one of the poles of the magnet in the telephone-preferably to the pole farthest from the telephone disk, a straight or "bar" magnet being usually the kind used in the instrument. The tuning fork is mounted on a sounding board. Similar apparatus is provided at both ends of the line. To sound the alarm, in order to give a call of the telophone, it is only necessary to draw a fiddlebow across one of the tuning forks; the other, at the further end of the line, responds with a note loud enough to attract attention throughout a large room. A Useful Little Article.-Very few persons can write a signature so that it can be read. We involuntarily get into the habit of writing our namesrapidly, and the result is that our correspondents cannot read them. Attempts have been made to obviate this difficulty by means of stamps, but hitherto stamps, if good for anything, have been clumsy, bulky, and the very reverse of portable. The latest thing of this kind, however, is a pocket stamp, which can be attached to a pencil. It is neatly mounted in a nickel-plated case, and proPolyzoa with "bird's heads," foraminifera, poly-tected by a cover, and the rubber stamp gives a cystina, holothurian plates, synapta anchors, chiro- clear, sharp impression, which can never be misdota wheels and young; spicules of sponges and Slides of various kinds of fertile fern fronds, Indian corn, deutzia, citra odora and other leaves, decolorized and stained, exchanged for other good microscopic objects. C. Blasdale, M. D., Jericho, Queens County, N. Y. gorgonia, etc., in exchange for foraminifera, path- interpreted. Another advantage is that they are ological, anatomical and vegetable preparations. very cheap. Address Scott & Co., 291 Broadway,. F. M. Hamlin, M. D., Auburn, N. Y. New York. Hints on the Selection of a Physician's rule that will apply to all. Let us take the following as a typical letter for consideration: Microscope.* "Dear Sir-I have read some of your contributions to the AMERICAN JOURNAL OF MICROSCOPY. * * **I am a physician URING the past of ten years practice; am located in a town twelve years, the of some ten thousand inhabitants. I am author has received satisfied that I ought to know enough about hundreds of letters microscopy to enable me to examine intellifrom as many indi- gently urinary deposits, cancerous growths, viduals commenc- etc., and to this end do I propose to puring the study of chase the necessary equipment. Any inforthe microscope, de- mation that you may be pleased to give me siring such infor- will be with pleasure received," etc., etc. mation as to the Now, I would answer this letter, and did best way of making answer it, thus: When you buy a stand, get an investment as one that you will have no occasion to sell at may pertain to the a ruinous sacrifice. I recommend that you selection of stand, objectives, etc. The purchase one of the cheap and moderate said letters have met with a prompt re- priced instruments, and at the same time sponse, and, as has before been stated, one that will do any and all work. Such a perhaps no two of these have been alike. stand ought not to cost, with one eye-piece, In each and every case there has been some more than $35 (and such are described in dissimilarity of circumstance, or, on the this little book). Now, as to objectives; other hand, we have suffered some change you can do all the work named with a one in our own views. Be all this as it may, the experience of the last two years enables us to speak more precisely to the point than before, and we now endeavor to respond to the interrogatory, "What shall I procure for an outfit?" There can be no general *Taken from the advance sheets of "How to See with the Microscope." By Prof. J. Edwards Smith. New York: Industrial Publication Company. inch of tolerably low angle, costing you, say, some $7; and a real good 3-10ths of 70° aperture, which will cost you, say, $11. The chances are, however, that as you become familiar with the use of the instrument, thereby learning its value to you in your daily professional practice, you will feel an inclination to dip somewhat deeper into the problems which will most assuredly surround you. In your examination of or two years, my correspondent will either urines, you will get glimpses of bacteria; push his own observations, or desire to keep you may meet with structures which you pace with those of others, and over strucare almost assured are "pale hyaline" tube tures of the most delicate and "difficult" casts, and you naturally desire a little more character, and now he will need a 1-6th or amplification and definition thereon, to en- a 1-10th, of the widest possible aperture. able you to pronounce with certainty. In Allowing this to occur he will have exfact, you now want such a glass as a dry pended but $110, which is less than the 1-4th of 110° or 120°, and adjustable. Now, usual cost of a nominally first-class 1-16th, to use this glass to advantage, you have and all the glasses on hand will still be of first to become familiar with its manipula- service. Besides all this, he is well armed tions. It will be requisite that you arrive and equipped for any work requiring at some knowledge of this before such an powers from 50 to 5,000 diameters, and objective can be of much avail to you. there will be no objectives on hand that will Now, if you purchase the 1-4th recom- not be worth their cost. mended, you will still have room for the employment of the 3-10ths, but you will On the Measurement of the Angle of Aperhave to study the use of the 1-4th, just the same as if you had never seen an object-glass; hence I had rather recommend that you purchase the 1-4th at the start, and thus get early accustomed to the use of adjustable glasses; and in this latter case it will almost has given any results at all approaching to assuredly occur that you will eventually desire to employ glasses of the widest apertures, and the experience you have gained with the use of the 1-4th will be of the utmost value to you. Moreover, the 1-4th, too, will continue to be a useful intermediate glass. And thus, in replying to all my correspondents, I recognize the importance that one and all shall early become acquainted with the manipulations of adjusting glasses. To accomplish this they must use objectives of the first class and which are reliable, i. e., which will respond promptly to change of collar adjustment, keeping well in mind the importance of buying nothing to be discarded, or thrown out of use in the future. In the case under consideration it will happen, in nine cases out of ten, that in less than two years the buyer will feel that he needs a first-class inch, or perhaps a 2-3ds, in which event the old inch will be of great service as a sub-stage condenser, providing that the stand has been selected with this end in view. It may further happen, in truth it will be likely to happen, that, in the course of one ture of Microscope Object-Glasses. BY F. H. WENHAM. HAVE long held the opinion that no method of measurement hitherto used accuracy, and in many instances the angle of aperture of microscope objectives has not reached even to half the number of degrees attributed to them. The mistake arises, as I have repeatedly urged, from the entrance of oblique pencils or rays extending in a direction away from the central or axial one, so that it is the angle of the extreme rays at the margin of the field of view that we measure, instead of the angle proper. In order to confine the apex of the cone or pencil of rays to one central point in the axial focus, I proposed that the sector measurement should be performed with a very narrow slit or minute perforation set precisely in the focus; but this, though right in theory, was found to be difficult and impracticable for high powers, as even the narrowest slit that could be used must have a diameter causing an error of angle to be indicated greatly in excess, and it was impossible to assign the exact limit of angles because no distinct image could be seen through a minute slit. In order to obviate this objection, and obtain a result that should be consigned to an exact centre line, a half screen bisecting the field in the definite diameters that could only admit the focal plane was next proposed, the screen base of a cone of rays, from an angle taken to be shifted alternately during the meas- from the axial focal point, up to a known urement. But this plan was abandoned as diameter of stop. The arrangement that I altogether fallacious, for it is the property use is a form of adjustable slit, consisting of a lens that every open point on its work- of two strips of very thin platinum foil. ing surface admits a ray from every portion One piece is cemented on to a parallel slip of the field of view. The effect by obscur- of thin plate-glass, which is made to slide ing the alternate halves is simply nil, the under two staples by a micrometer screw sum of the two measurements being equal acting against a counter spring. The fixed to that of the entire uncovered lens. I strip of foil is attached to one of the staples, mention this because Drs. Nickolls and so that when the screw is quite home, the Bull suppose that as the result corresponds edges meet. The various widths at which nearer with the open measurement, it must the instrument was set were measured be considered accurate. under the microscope with eye-piece miI next proposed to eliminate the mar- crometer. Having got the desired width, ginal pencils of the field by means of a the object-glass to be measured was atsmall stop set in the focus of an eye-piece tached, and the body of the microscope adaptation. This has also been found to lowered till the slit came in contact with give unreliable measurements, but still the front lens, a drop of water having been it affords no disproof of the accuracy of placed over the slit, to prevent undue rethe principle to which I have called no- fraction and obtain more light. tice. Of this I now give additional demonstration. The apparent angles included by these different limiting edges or stops were measI selected a 1-5th of fine quality, which ured by the usual sector method of rotating worked as an immersion. This position the microscope on a turn-plate, graduated prevents quibbling concerning various posi- into degrees, and ascertaining the vanishtions of adjustment. The aperture, as ing point of a distant light, or preferably measured by the sector through a slide, with by means of an examining lens at the eyewater between that and the front lens, was piece, so as to observe the disappearance 120°. The focal distance as immersion was from the field of an actual image. .041 (I use a Zentmayer form of stand, which is fitted with a micrometer scale and vernier directly on to the fine motion slide, so that depths can be measured very accurately). The diameter of transmission on the surface of front lens was .07. This is ascertained by allowing a drop of milk to dry on the front, and measuring the diameter of the light spot from parallel rays, entering the back, using a low-power objectglass and micrometer eye-piece for the measurement. The real or true angles were estimated from the distance of the focal point, up to the known measure of the edges of the stop. Avoiding fractions of degrees, the following table gives the comparative results:— Working diam. of front lens. False aperture. True aperture. ⚫07 120° 89° ⚫06 118° 80° ·05 116° 70° 04 103° 58° ⚫03 88° 45° ⚫02 The field of view included a diameter of .03 on a stage micrometer. Now, having The last item, with a stop 1-500th of an the exact focal distance from the surface of inch in diameter, indicates an aperture of the front lens as a starting point, it re- 50 degrees by the usual methods, whereas mained first to ascertain what were the ap- the true angle is only 3 degrees-an error parent apertures taken through various being shown in excess of more than foursized stops set close on to the front, having teen times. The annexed diagram demonstates the I have prepared a slide for the Society's cause of this error. The central angle collection, in which the gold is mounted shown is the true aperture assigned by the dry, after first partly removing the impuriwidth of the small stop at the base. The ties with which it was mixed. It will be oblique angles represent the pencils includ- seen, however, that there still remains coal, ing the field of view, and showing light, or sand, fibres of wood and cotton, hairs, etc. In washing out the extraneous matter, the finer particles of gold floated on the surface of the water, and I found it impossible to cause them to sink, although I tried every plan I knew of. This experiment would seem to prove the truth of the oft-repeated statement of miners, that much free gold escapes them from this cause. 100 03 -002 50 an image, in the eye-piece up to an angular The Divisibility of Gold. HAVIN On examining the slide under the microscope, it will be seen that the gold is invariably in the form of scales, and much resembles the natural gold scales found in the beach sands at Cape Blanco, except that in the latter the edges and faces of the scales are rounded by attrition in being washed about by the waves. It has been a source of wonder to gold miners that nearly all placer gold is found in flat pieces. Some ingenious theories have been advanced to account for it. The sample I present this evening would seem to indicate some law which governs the form of its fragments. I admit I cannot clearly understand how rounded masses of metal like gold coin should react on each other mechanically and produce flat fragments, as shown in the specimen. To give an idea of the size of the particles of gold, I have measured some of them in decimals of an inch, and I find them to vary from .003x.001 to .0425x.0235. A paper read by Prof. H. G. Hanks before the San Francisco Microscopical Society, Jan. 17, 1878. [AVING recently had occasion to refer to statistics showing that gold coin loses an appreciable amount even in a single handling, and being desirous, in connection I have also measured a large number of with my investigations, to examine micro- the gold scales from Port Oxford, and find an scopically the gold dust resulting from such average of .0095 mean diameter by .00374 abrasion, I asked Mr. L. A. Boynton, of the mean thickness. The samples were washed United States Treasury, of San Francisco, ashore from the beach sands found a few to brush out the wooden tray in which large miles south of Cape Blanco, on the coast of sums of gold coin are daily handled, and Oregon. Ten of the scales, selected at ranwhen he had accumulated sufficient to see dom, and melted into a button, weighed the gold, to send it to me. He has kindly 0.0021327 grammes. From this weight I complied with my request, and I am able have calculated the number of such scales this evening to call your attention to some which would be required to make up the interesting facts which have developed weight of an ounce Troy. themselves in the examination. If I have made no mistake in my figures, |