the objective. With the highest objectives generally used with black ground illumination, as a 4th of 75° to 110°, the object seems no brighter than usual, but the field is free from the foggy diffuse light, otherwise present, and the object appears, beautifully distinct, upon a jet black ground. Even a 4th or 4th of 130° gives the same effect of a deep black background, and shows the object with good stereoscopic effect in Wenham's binocular. With ob jectives of 170°, the main effect is that of a dark background, though not so perfect as with the lower angles.-T. D. B.

BICHROMATIC VISION.-Mr. J. W. Stephenson, inventor of the recent binocular microscope which bears his name, has noticed that if different colors are presented, simultaneously, to the two eyes, the sensation produced will be that of neither of the two colors, but of one which would be produced by mixing them together. If the colors presented are strictly complementary, the effect will be that of common white light; as the two bright colored disks produced in the field of a microscope by a double image prism and a selenite plate, become white where they overlap. The effect is best studied with the binocular microscope and polariscope. A plate of selenite is introduced so as to give both fields of a bright conspicuous color; and then a film of mica is interposed in the course of the rays supplying one tube, of such thickness and position as to give, by retardation, a color as nearly as possible complementary to the first. One field, for instance, may be a bright red, and the other a bright green, while the observer, viewing both at once, will see only a colorless field. By an ingenious changing of the plates by which the colors are produced, both fields may be gradually changed to totally different colors, the complementary character being maintained throughout the change, without any knowledge of the change on the part of the observer. If the color of one field is entirely removed, the observer becomes slowly and feebly conscious of the color of the other. The optical and physiological bearings of this discovery are obvious and interesting.

NEW ARRANGEMENT OF SPRING CLIPS.- Miller Bros., of 1223 Broadway, N. Y., are manufacturing a contrivance which must be, for certain purposes, a very convenient substitute for Dr. Maddox's spring clips. It consists essentially of a mahogany strip, of suitable size, grooved upon its upper surface and protected with

pins in such manner that a dozen slides can lie, side by side, securely upon it. An equal number of thin brass wires spring from one side of the block, and are bent down so that they can be easily made to press upon the centres of the covers, to hold them in position while the balsam or other mounting material is hardening. Little cork disks are furnished to place upon the covers and beneath the springs. For some uses the corks would doubtless be dispensed with, and when needed they would probably be more convenient if attached to the wires by passing the wires through them. An additional groove should be cut in the wood under one end of the glass slides to facilitate the removal of one slide without disturbing the others.

SINGLE FRONT OBJECTIVES.—Mr. Wenham believes that the principal use of the late discussion upon the working angular aperture of immersion objectives viewing balsam-mounted objects, which angle he still maintains is necessarily limited to 82°, although Mr. Tolles cannot see the difficulty of its exceeding that figure, consists in the dissemination of the information that the best American objectives, both dry and immersion, are now made with single fronts. As the originator of this style of construction, though having at the time no knowledge of its importance nor expectation of the success it has since attained, he naturally feels an undisguised interest in its success. The triple-front objectives he considers already obsolete.

MICROSCOPY AT THE AMERICAN MEDICAL ASSOCIATION. During the Philadelphia meeting of this society, this summer, an evening reception was given at the Academy of Natural Sciences, at which music and sociability were supplemented by the entertainment afforded by microscopic specimens. One hundred microscopes were used, and novel accessories exhibited.

STRUCTURE OF DIATOMS.-Prof. Adolf Weiss, of Lemberg, has published some researches upon this well-studied but still obscure subject. He regards the silicious envelope as capable of polarizing light, and as consisting of a cellulose coat more or less infiltrated with silex. He does not consider the individuals one-celled, but finds the valves composed of cells from .008 to .00025 mm. in diameter. These cells are furnished at their centres with papillæ which appear as striæ under low powers and as moniliform mark

ings under high powers. The large cavity between the frustules is regarded as equivalent to the embryo-sac of higher plants, and the formation of new individuals has been observed within it. An alternation of generation is indicated by the observations made.

ORIGIN OF CANCEROUS DEPOSITS.- Dr. J. J. Woodward discusses this question in a report to the Surgeon General. His observations of structure do not differ materially from those of other recent observers, though the cell walls of the cancer cylinders, described by Koester, he is able to detect in only a portion of the cases. He reviews the theory of Koester who regards the nucleated cylinders as transformed lymphatics, and of Thiersch who explains them as outgrowths from the lower layer of the epidermis and from the epithelium of the glandular apparatus. The latter view was originally applied to epithelial cancer, but has been extended by Billroth to cancer generally. Dr. Woodward is manifestly unwilling to commit himself to any theory, but rather favors Koester's on account of the well known similarity of the morbid growths when affecting different organs, and on account of the manner in which the cell cylinders anastomose, which points rather to the lymphatics than to the gland tissue. He seems not unwilling to regard the cancer cylinders as consisting of transformed white corpuscles accumulated in the lymphatic passages. The presence or absence of a cell wall he justly considers unimportant, it being only an indication of age in cells which, according to our present knowledge, consist originally of only a nucleus embedded in a mass of protoplasm.

THE "NERVE" OF THE TOOTH.- Mr. T. C. White has read a very interesting paper on this subject before the Queckett Microscopical Club. Though considering it a painful subject. and not to be touched upon except very lightly, he nevertheless considers it interesting to know something of its structure and

uses.

The pulp, or so called nerve of a tooth, should be obtained from a tooth of the temporary set removed in a state of health to make room for the advancing permanent set. A longitudinal groove is to be filed around the tooth, which is then to be very carefully washed, and then split with a pair of wire nippers. The pulp will thus be fully exposed, and may be stained by soaking for twenty-four hours in an ammoniacal carmine solution as recom

mended by Mr. Beale, washed, soaked in glycerine for a few hours, and finally flattened by gentle pressure in a compressorium for a few hours more until it is sufficiently thin to be examined by ath inch objective. It is also advised to soak an entire tooth for a few weeks in the carmine staining fluid, then decalcify it by immersion in hydrochloric acid, and cut thin slices through the whole which will show the pulp and decalcified osseous tissue in their natural relation to each other.

Thus studied, the "nerve" appears to be a mass of areolar or connective tissue, through which ramify the nerve, vein, and artery. It not only constitutes a very delicate sensory organ, but originally was the means of building up the dentine; and even in adult life performs an important part in sustaining the vitality of the tooth, and is capable under certain stimulating influences of developing dentine again. [The unsatisfactory nature of a tooth whose "nerve" has been "killed" would seem to be confirmed and explained by these views of its functions.]

MISNAMING OBJECTIVES.-[Although the controversial part of this question has occupied too much time already, we publish the following note from Mr. Stodder who seems entitled to an opportunity to correct the idea that his having previously written over initials implied an unwillingness to assume full responsibility for his statement. The editors of this Journal are not responsible for anything credited either by name or initials to any other authority.-EDs.] The brief remarks of mine, printed over the initials C. S. in the March number of this Journal, were copied essentially in the "Monthly Mieroscopical Journal" for April. In the May number of that periodical Mr. Wenham writes a reply. It is a remarkable paper not only from the eminence of the writer, as an authority on microscopy, but from his evident loss of temper and by the terms to which he refers to Mr. Bicknell and to C. S. Under these circumstances I must ask for a little space for a rejoinder to my share; I have nothing to say for Mr. Bicknell as he is able to take care of himself.

Mr. Wenham commences his paper which he calls a "reply" with this," to correct a misstatement that I [Mr. W.] wrote a paper in reply to one of Mr. Bicknell's; I did not commit myself to such an extent." This is a mere quibble, unworthy of its author. The very caption of the paper had Mr. Bicknell's name

in it. I should not have noticed this, had not Mr. W. unfairly, as I think, charged me with a misstatement.

Next, Mr. Wenham couples C. S. and Mr. Bicknell together as if they acted in concert, and were joint writers. I can assure Mr. Wenham that it is not so. Mr. B. is not responsible for any thing I have written, nor am I for him. Neither had seen the other's writing until it was public.

Next, I have no "plea or atonement" to make "for expressions hastily or inconsiderately written." My expressions were used deliberately and after full consideration of their import. I still hold the same opinion, namely, that selling an objective by a name that does not approximately indicate its focus (i. e. 1 for 1,2 for or, as I have known, 2 for, as in the case of an eminent French maker; or, as in another instance, a for a; or, as in the case of an English objective that I have recently heard of, a for a 1) is an "imposition," or a fraud if that term is preferred, not applying it, however, as Mr. Wenham represents, to a particular firm, but to all, of any country, who practise such "imposition;" and that Mr. Wenham in his paper, by stating that "ths were 4ths or ths, and some now approach ths in power," without disapproval, was practically defending the custom, and that he does not now deny. His paper in reply to Mr. Bicknell was published in December. In May he writes, "no one knows better than myself the difficulty of adopting a nomenclature that shall exactly denote the power of all the highest object glasses sent out"-something has evidently produced some effect on him since that time. The complaint was not of want of "exactness," but of gross misnamers of twenty or fifty per cent., such as he named in the December paper, not in regard to the highest powers alone but applicable to the lowest powers as well. Such was what I called an imposition, and I call it so now.

In the "Quarterly Journal of Microscopical Science," October 1862, Capt. Mitchell gives the measurement of the focus of several London objectives; most of them being undernamed. Capt. M. complains of this; he says "when I buy a 4th, I want a 4th, not something else." He calls those correctly named, honest; by implication, those not so named, dishonest.

Dr. Wm. B. Carpenter ("The Microscope," fourth ed., 1868, p. 184) says, "the designations given by the opticians to their objectives are often far from representing their focal length, as estimated by that of single lenses of equivalent magnifying power, a