the test was sufficiently difficult. On the whole, the result seemed to him quite satisfactory, but there was a certain muddiness in the picture, due to the use of an old amplifier which needed repolishing. He was having some new amplifiers made with which he expected to get more sparkling pictures.' The photograph accompanying the letter was handed round for examination; it showed the striæ with remarkable distinctness. Mr. Ingpen inquired if any question as to priority in regard to the invention or use of this prism arose out of the paper. He had himself seen a prism with four faces, made by Messrs. Powell and Lealand, from drawings by Dr. Edmunds, which appeared to more than cover the ground aimed at by the prisms now shown. Dr. Edmunds, in reply to a question from the President, said that, as to the prism to which Mr. Ingpen had referred, he had not thought it of sufficiently marked novelty to make it the subject of a formal communication, but he would be pleased to submit it to the next meeting. Upon the question of priority, it would be recollected that in 1856 Mr. Wenham described a small right-angled prism, which he attached to the under surface of the slide with oil or balsam, and used for oblique illumination. This application of a right-angled immersion prism was clearly due to Mr. Wenham. What was due to Colonel Woodward in his beautiful work was not the invention of the right-angled prism, but its combination with a pin-hole shutter, through which a small beam of parallel light could be thrown into a balsam-mounted object outside the angle of 41° from the optic axis; thus demonstrating that pencils outside 82° balsam-angle could be used to form an image if only the objective were of sufficiently large aperture. With reference to his own prism, he had had the advantage of working with that sent over by Colonel Woodward, and finding it difficult to manage, and that its corners practically prevented its rotation under the slide, he had had another form constructed by Messrs. Powell and Lealand, and this had proved most useful and very easily managed. It was made from rather more than a hemisphere of glass, of which the spheroidal surface was ground down into four faces making practically two right-angled prisms at right angles to each other, one having its faces inclined to the plane surface at 41° and 49°, and the other at 30° and 60°. The prism was set in a simple brass tube with a slot for each face, and fitting below into the substage. The faces were so arranged that an object in focus was illuminated equally through each face by light entering it at the normal. The upper surface was inch in diameter and was made optically continuous with the slide by oil or glycerine. By simply turning the substage, the prism gave unrefracted light at 30°, 41°, 49°, and 60° from the optic axis, and each angle could be varied a little without practical detriment. The light reflected from the top surface of the prism passed out through the other side at the normal, and thus glare was prevented. The prism worked charmingly. Mr. Crisp said that the interest of Colonel Woodward's present apparatus arose entirely out of the angle of aperture discussion. Colonel Woodward certainly had never claimed and did not intend to claim the invention merely of a right-angled prism for illuminating objects under the microscope, but obviously the particular application of such a method for the special purposes dealt with by his paper. As Dr. Edmunds had said, the right-angled prism was undoubtedly not "new"-indeed it had become very difficult to substantiate a claim to actual novelty as regards the mere form of any illuminating apparatus, as things seemed to be re-invented over and over again in more or less regular cycles. Mr. Wenham, who was admittedly the "first and true inventor "of the right-angled prism, used it, however, entirely for the illumination of objects under high powers by total internal reflection from the upper surface of the covering glass. Those curious in the matter would find a summary of the various analogous devices of Mr. Wenham and others in the second German edition of Harting's 'Das Mikroskop,' the figures in which he showed to the meeting. Canada balsam or other medium was interposed between the prism and the slide in Mr. Wenham's arrangement for the purpose of preventing the rays from being reflected from the back of the prism instead of passing on to the covering glass. Mr. Crisp (Secretary) said that a Congress of American Microscopists had been held in August at Indianapolis, at which the subject of a "Unit of Micrometry" had been taken into consideration, and he read the Resolutions which had been passed by the Congress recommending the use of the of a millimetre (see p. 254). Mr. Beck inquired if it was intended to take any action with reference to the Resolutions. The desirability of a uniform standard had always been appreciated in this country, and it would be a very desirable thing if the Society would now give the weight of its influence towards the establishment of such a standard. Some time ago the adoption of a single thread for the screws of all objectives was due to the action of the Society, and though perhaps the one decided upon might not be the best, yet the adoption of a uniform gauge had been of very great advantage in the way of getting rid of those horrible nuisances, the adapters. He merely threw out the suggestion because he thought the idea was worth their consideration, and he should like to know whether or not the Council were prepared to take any action in the matter. Mr. Crisp added that at the Congress a paper had been read by Professor W. A. Rogers, of Harvard University, in which it was understood that he had explained the application of a machine he was using for the production of "standard" micrometer scales, and by means of which he believed that he could rule any number of micrometers precisely alike and with exceptional accuracy. It should not be forgotten that, as Mr. Reeves reminded him, the subject of a uniform micrometric standard had been very completely dealt with by Dr. Cooke in a paper read before the Quekett Microscopical Club, and printed in their Journal on p. 1 of vol. i. Mr. Beck, in reply to the President, said there would be no difficulty whatever in obtaining scales ruled as required for the adoption of the proposed standard. Dr. Edmunds thought that the whole question of microscopical measurements wanted going over and revising. It had been said that there was a Society's screw for objectives, but when he wrote to the Assistant-Secretary to ask him for a standard gauge, he was told he could not be supplied, and so he found that his mounts of Powell and Lealand's would not fit Ross's, and that Smith and Beck's again differed from these. Mr. Beck said, there certainly was a standard in the possession of the Society, which was made by Whitworth at the time the question was settled, and there could be no doubt that by applying to Whitworth they could get others so exactly the same that all made to it must accurately fit one another. The differences now complained of were owing to makers not having revised their screw tools from time to time, so as to ensure that they were accurate. Dr. Edmunds said that there were also other questions that should be pronounced upon by the Council. For instance, what is the " 10-inch " tube? In practice the length varies with each of the makers, and the eye-pieces are so constructed as to destroy all uniformity in the datum point at the eye end of the tube. Practically the eye end of the tube measured from the diaphragm in the eyepiece. Nevertheless the 2-inch eye-piece-one of the chief makers only excepted-was shouldered about an inch beyond its diaphragm. On the other hand the -inch eyepiece necessarily has its shoulder about on a level with its diaphragm. It followed that in changing the 2-inch eye-piece for the -inch the optical tube was lengthened no less than an inch. Therefore the amplification given by the objective at the point where it was taken up by the eye-piece, was greatly increased while the objective itself needed to be readjusted, and to have its focus shortened before it was fit to be viewed by the -inch eye-piece. Yet all this could be remedied by the simple plan of shouldering the eye-pieces on a level with their diaphragms. Then again, why should not the visual and substage tubing of the large microscopes always be of the same size? At present several of the chief firms made their substage tubing different in gauge from their own visual tubing, and consequently the eye-pieces could not be used for condensers. Nor was there any adequate reason why all the large microscopes, and on the other hand, all the small microscopes, should not be made to a standard gauge, so that apparatus made by various makers could be used indiscriminately. The President said that as to the standard of the Society's screw, that was carefully kept in the possession of the Council, and copies could no doubt be obtained from Whitworth if they were required. With regard to the present discussion, it would be understood that Mr. Beck would be kind enough to give them his ideas on the subject of micrometric measurements, and that Dr. Edmunds would similarly undertake the other questions as to size of screw, tube, &c. A letter was read from Mr. F. Habirshaw, of New York, in reply to one addressed to him by Mr. Crisp in reference to the discussion that had recently taken place on the "Revification of Diatoms," and particularly the note on p. 150, in which Mr. Habirshaw said, "Without doubt, the diatoms were alive. Nothing was used but filtered water from the ship's tanks, and I have not the least doubt as to their revification. Captain Mortimer is an accomplished naturalist, and not a man to make idle assertions, or be easily led towards error." Mr. Crisp described and exhibited (1) the "Miniature Microscope" of Mr. Browning (see p. 296), intended as a more powerful substitute for a hand lens, and capable of holding ordinary slides. The one exhibited, which was in nickel silver, was examined by the meeting. (2) The "Transporteur Monnier," which was said to remedy the annoyance occasioned by displacement of an object (which had been carefully arranged in the proper position in glycerine, &c.) when the covering glass was put on (see p. 265). Whether there was any such an evil to be corrected, those who were more experienced in mounting than he was would be able to say. (3) An objective made by the Bausch and Lomb Optical Company, of New York (under Mr. Gundlach's patent), with the " new correction adjustment (see p. 252). Mr. Ingpen undertook to examine the objective, and to be prepared to say something on the usefulness or otherwise of the addition at the next meeting. The President announced that the first of the "Scientific Evenings" for the present session would take place on Wednesday, 27th November, when he hoped the Fellows would assist in making the meeting as interesting as the previous ones had been. Donations since the June meeting (exclusive of exchanges) : American Journal of Microscopy, April to September Fischer, Prof. A., Le Microscope Pancratique, 1841 From The Editor. Author. Ditto. Journal of the Cincinnati Society of Natural History. Vol. I., Nos. 1 and 2 Memorias da Academia Real das Sciencias de Lisboa for 1875 .. Orth, Dr. J., Cursus der normalen Histologie, 1878 Sessão Publica da Academia Real das Sciencias de Lisboa, 1875 1877 .. Academy. Proceedings of the Geologists' Association. Vol. V., No. 6 Verhandlungen des Naturhistorisch-Medicinischen Vereins zu Heidel- .. Society. Club. The following gentlemen were elected Fellows of the Society :Major A. Morton Festing, F.M.S., Staff Paymaster Army Pay Department; John Borland, Esq., Member of the Pharmaceutical Board of Examiners for Scotland. WALTER W. REEVES, Assist.-Secretary. JOURNAL OF THE ROYAL MICROSCOPICAL SOCIETY. DECEMBER, 1878. I.-On a Species of Acarus, believed to be new to Britain. (Read before the ROYAL MICROSCOPICAL SOCIETY, 1st May and 13th December, 1878.) PLATE XVI. Cheyletus venustissimus. In the beginning of January last (1878) I found this species, which I believe has not hitherto been detected in Britain, in a stable; near Tamworth I noticed one or two minute red specks running rapidly over the fodder. I secured three or four for examination, and I then ascertained that it was the Cheyletus venustissimus of Koch;† that author, in his work published in 1839, describes and figures the species in a manner sufficient for identification, although somewhat wanting in detail; he states that it was then found in the neighbourhood of Regensburg and in Rhenish Bavaria, and was somewhat rare. The species is not noticed by Walckenaer and Gervais, and I am not aware that any subsequent author has noticed it, except by stating that Koch gives it as one of the genus; as Koch's notice is not very full and does not mention the larva, pupa, &c., a description may possibly be acceptable. EXPLANATION OF THE PLATE. FIG 1.-Under side of female x about 70 (the imbrication of the hairs is somewhat exaggerated); a, horseshoe-shaped ridge round anus. 2.-Rostrum and palpi x about 200; a, first joint of palpus; b, second ditto; c, falx; d, third joint of palpus. 3.-Larva. 4.-Foot highly magnified. Subsequent to the reading of this paper Mr. McIntire was good enough to show me a slide of unnamed Cheyletus captured by him. It has been mounted some years, but I think it quite possible that it may have been this species. + Deutschland's Crustaceen,' Heft xxiii. p. 22. VOL. I. Z |