of lines. If this has been anywhere stated, I should be glad to learn it, but my counts were all made in perfect ignorance of any enumeration but my own. Nobert, to whom I sent the photographs with my count, acknowledged the resolution in handsome terms. If there are any persons who still remain unsatisfied, I much hope that a better acquaintance with the subject will lead them to modify their opinion.

I pass by here the question of intermediate spurious lines between the real ones, since this concerns chiefly the lower bands of the plate.

It has, however, been suggested that if the criterion I have proposed be accepted, it ought to be applied also to the diatoms, and that it would invalidate all claims as to the resolution of these which are unaccompanied by a count and actual measurement. As to this, I would say that the optical conditions in the case of the diatoms are so different from what we have to deal with on the plate, that I cannot see that the one conclusion follows from the other.

The lines of the plate are minute grooves on the under-surface of the thin glass cover, and the point is to distinguish them from the spurious images to which they give rise. The striae of the diatoms. are the optical expression of sculpturings on frustules of silica. The appearance of lines is now generally conceded to be an illusion. What seem to be such are generally the optical expression of minute elevations, most probably hemispherical in shape, though the question of their form cannot be regarded as settled. These elevations are arranged in rows, to which the apparent striæ correspond. False lines of greater or less number are occasionally produced, and have in some instances been described as real ones, but this does not occur with facility on the frustules of most species, and on many does not occur at all. The question of the resolution of the diatoms, however, is too complex for further discussion in this place.

In closing this paper, I trust I may be pardoned a few remarks which appear to me to be warranted by the tone of Mr. Stodder's reclamations. He is quite right in his allegation that I have done "something more" than ignore his claims. I controvert them. I do so, first, because I have carefully tried a number of objectives made by Tolles, and have been unable to see with them any but spurious lines in the nineteenth band. Among those which I have tried is the much-talked-of th and the new 13th belonging to Dr. Josiah Curtis. Secondly, because Mr. Stodder has never yet offered any sufficient evidence that the lines he saw were not spurious also. He rests on a simple supposition, and in his recent paper supports this supposition by the mere opinion of several gentlemen to whom he has shown lines in the nineteenth band, but who, like himself, have taken no precautions to determine whether the lines seen were spurious or real.

I have never, however, expressed a doubt of Mr. Stodder's good aith in his claims, and will not do so now. Still I must call attention to some inadvertencies into which he has fallen. Thus on page 120 of his article he passionately denies having made any error as to the matter of counting fine lines, and quotes in proof the passage in his original paper, leaving out the part which contains the error. The whole passage reads, "either the micrometer or the stage must be moved, and it is next to impossible to construct apparatus that can be moved at once the Toooooth part of an inch and no more." His error, of course, consisted in supposing that if the micrometer is moved, its motions must correspond with the real distance of the lines, instead of the dimensions of the magnified image. The remarks on tremor which he introduces in this place have nothing to do with the question.

On the same page he insinuates that I have misrepresented the meaning of Professor Hagen's paper, but here his confessed ignorance of the language misleads him. I quote a single passage in reply. "Bis jetzt keines der objective von Tolles die 16 bis 19 Bande in Nobert's Platten vollig auflöst, was mit eth von Powell und Lealand gelungen.' I might quote several other examples of what I must hope is unintentional unfairness, especially his criticism of my photographs, of which I will only say that it contains conclusive internal evidence that he is unacquainted with the appearance of the true lines of the nineteenth band. But I am quite willing to leave this matter in the hands of conscientious students of the plate, and have neither time nor inclination to discuss his errors seriatim. I will close by a brief reply to his demand for my opinion as to Tolles' lenses.

I have always felt great admiration for the excellent workmanship of Mr. Tolles. I think his ths and ths will compare favourably with the like powers of the best makers, but I have not found that they excel them, and regard the claim that the 4th of Tolles will do the work of the th, or his ths that of the ths of other makers as utterly unfounded. I have long thought that if Mr. Tolles would apply himself to the construction of an immersion lens of shorter focal length than those he has hitherto made, the result would be gratifying to his warmest friends. Some time prior to the appearance of Mr. Stodder's paper, therefore, I sent through him an order for such an objective. When it reaches me I will endeavour to do it full justice; in the meantime I reply directly to Mr. Stodder's question, that I have two ths by Powell and Lealand now in the Museum, each capable of resolving the sixteenth band of the plate, which is all I have ever been able to do with Tolles'th or th. With this I take leave of the subject. * Max Schultze's Archiv,' Bd. vi., p. 217.

VII.-On the Employment of Dammar in Microscopy.

By Prof. ARTHUR MEAD EDWARDS, New York.

In the London 'Quarterly Journal of Microscopical Science' for January, 1871, appeared an extremely interesting and valuable paper by Mr. Henry N. Moseley, "On the Use of Nitrate of Silver and Chloride of Gold in Microscopy," in which he calls attention to the use of "Dammar-firniss by Stricker in place of Canada balsam as a medium with which to mount objects, the more especially histological preparations. And it is remarked that in this, Stricker's, laboratory, as well as in those of Brücke and Rokitansky, this medium has entirely supplanted Canada balsam. Mr. Moseley points out that well-made Dammar varnish possesses several advantages over the microscopist's old friend Canada balsam; and proceeds to point out that it is "clearer, more free from colour, and when used cold, as it always is, it dries quicker, though it is much thinner and more limpid." He also remarks upon the difficulty of obtaining good Dammar varnish in London, although the gum from which it is prepared is common enough.

As I have had some experience in the use of this material in microscopy, I will take the liberty of transcribing a paper read by me on this subject before the American Microscopical Society, April, 29, 1865, and which has never been in print as yet. Hereafter I will give some of my later acquired knowledge in this connection. The paper is entitled

On a New Material for Mounting Microscopic Objects.

"Although I have called the material for mounting microscopic objects, which I am about to describe, new, it may not be so to some of the many students of the microscope; but, so far as I have been able to ascertain by inquiry among our own immediate members, it has not been as yet brought into use in this country; and as I am of opinion that it possesses in some respects superior characters, fitting it for the special purpose to which I have applied it, to Canada balsam, I venture to bring it to the notice of this Society, hoping that such of our members as will give it a trial will be as well pleased with it and obtain as satisfactory results as I have.

"I was more particularly drawn to ascertain if it might not be used in mounting microscopic specimens on account of a lengthened series of investigations undertaken for the purpose of ascertaining the improvements accomplished in the manufacture of modern objectives, and the consequent use of such glasses to determine, if possible, the character of the markings to be found upon the silicious cell walls of certain of the Diatomaceæ.

"Having then examined, with the various objectives which passed in review under my scrutiny, the specimens I possessed mounted in the two ways most commonly in use-that is to say, dry in air and in Canada balsam-it struck me that it would be well to try the effect of various media in assisting the performance of the instrument used; and, to that end, I mounted several specimens in different ways and in various varnishes and liquids, amongst which one may be particularized as very difficult to manage-that is to say, benzole. Amongst all the varnishes which I tried, I obtained the best results with a specimen of very old Dammar, which I was lucky enough to meet with in a small quantity, and which I was assured, by the person from whom I procured it, was of superior quality as a varnish, on account of its having been made some time, and at a period when spirits of turpentine and not petroleum naphtha was used in its manufacture; the latter material being used at the present time for dissolving gums and resins in varnish making, the war having rendered turpentine extremely scarce, as is well known.

"Old Dammar varnish, then, is the medium which I wish to recommend to the notice of the members of the microscopic fraternity; and as to the points in which it is superior to Canada balsam, I would state that its refractive power is such that markings which are with difficulty seen in balsam with a 4th objective are with ease brought out sharply and distinctly when mounted in the Dammar with aths. It also dries almost immediately and without the use of much heat; in fact, much heat is rather detrimental, and I find the best method of procedure to be to dry the specimens of diatoms upon either the slide or thin cover as is desired, although the latter plan is the best, and slightly warming, drop upon them a very small quantity of pure spirits of turpentine, and, before it has all evaporated but has permeated throughout the mass of diatoms, to add the Dammar and bring the cover and slide, both slightly warmed, together. When mounting a number of specimens (say a dozen or so), as soon as we have put the cover upon the last the first is ready for cleaning, which can then be done with a small bradawl so as to remove the superfluous varnish, and the slide finished with turpentine. For cleaning slides the so-called "camphene" is the best material, as it is pure spirits of turpentine. Another great recommendation, as I consider it, to the use of Dammar for mounting microscopic objects is its great toughness, never becoming brittle by age, as is well known to be the case with Canada balsam. Besides, Dammar is commonly much lighter in colour than such Canada balsam as is generally to be found in the shops. When mounting specimens containing cavities -such as the Isthmia-perhaps even a little more care is necessary when using Dammar than Canada balsam; but when the effect

produced is taken into consideration, I am sure that microscopists will be willing to spare a little more time and labour over their manipulations so as to procure a superior quality of specimens."

Since this paper was written I have had much more experience in the use of Dammar varnish in microscopy, and mostly in preparing specimens of diatoms; but, all things considered, I think Canada balsam is the best material to use for that purpose. But as a cement, Dammar ranks very high, and I have put up a preparation of it for our principal dealer in microscopic objects and requisites, Mr. Miller, and he has found it extremely serviceable in the fastening together of glass as in constructing zoophyte troughs and growing slides. So my fellow-member, Dr. Arnold, favours very strongly its use as a cement in anatomical preparations. After having used it for some time and experimented considerably with this medium I consider that the reason why the first specimen I had was so clear was that it was thick. Dammar of good quality dissolved in coal-tar benzole and concentrated is very clear, otherwise it is milky until it thickens on the slide. Canada balsam, and, in fact, almost all solutions of resins in essential oils-i. e. varnishes-can be readily bleached by a few days' exposure to the sun in a closely stopped bottle. They are then much improved for use in microscopy.

VIII.-Experiments on Angular Aperture. By R. B. TOLLES. THE following is given to illustrate the comparative available angle of dry and immersion objectives.

In the figure, "A" represents a plano-convex lens, nearly hemispherical, applied centrically to an objective at its front face. The objective used had an angle of over 170°.

When the hemispherical lens is thus applied to the objective an air space of course exists between the plane surfaces. On testing the angle only 80° (or at most less than 82°) was obtainable. Were the plano-convex removed, the angle indicated would be 170° upwards. This was verified at the time carefully.