ART. 1910 CONTENTS OF No. XVIII. IV. THE MAGNESIUM AND ELECTRIC LIGHT AS APPLIED TO PHOTO- V. REMARKS ON HIGH-POWER DEFINITION. By F. H. Wenham, Vice- VI. ON A NEW CRITICAL STANDARD MEASURE OF THE PERFECTION OF HIGH-POWER DEFINITION AS AFFORDED BY DIATOMS' AND NOBERT'S LINES. By Dr. Royston-Pigott, M.A., Cantab., Fellow of the Cambridge Philosophical and of the Royal Astronomical and Microscopical Societies of London; formerly 300 The MONTHLY MICROSCOPICAL JOURNAL is furnished free to Members of the Royal Microscopical Society. Non-Members can get it from any bookseller, Contributors must write their Names and Addresses on the first page of MS. All Communications to be addressed to the Editor, 192, Piccadilly. a 2 THE MONTHLY MICROSCOPICAL JOURNAL. [June 1, 1870. LONDON INTERNATIONAL EXHIBITION, 1851.-TWO PRIZE MEDALS. LONDON INTERNATIONAL EXHIBITION, 1862.-Two PRIZE MEDALS. PARIS INTERNATIONAL EXHIBITION, 1867.-THE GOLD MEDAL. THOMAS ROSS, OPTICIAN, MANUFACTURER OF MICROSCOPES, TELESCOPES, PHOTOGRAPHIC LENSES, etc., etc., etc., 7, WIGMORE STREET, CAVENDISH SQUARE, W. (REMOVED FROM FEATHERSTONE BUILDINGS, HOLBORN), LONDON, Has the pleasure of announcing that he is now prepared to supply a second ANTERIOR ARRANGEMENT to his in. and in. Object Glasses, which, when substituted for the ordinary front combinations, convert them into IMMERSION OBJECTIVES, giving most brilliant definition. This addition, which is strongly recommended, thus enables the Microscopist to use these Powers either with or without water. The Price of the Immersion Arrangement for the in. is £2; for the in. £2.10s. ROSS'S 4-INCH AND 5-INCH OBJECTIVES, For the Examination of large Specimens of Mineralogical, Botanical, and Entomological Objects. These Low Powers, which have been constructed more particularly to use with ROSS'S TANK MICROSCOPE, are readily convertible into very convenient ACHROMATIC POCKET LENSES, Price of 2 and 3 inches focus respectively, by screwing the Optical arrangement into a German-silver Pocket Frame, specially made for that purpose. of the Frame, 20s. CATALOGUES FREE ON APPLICATION. THE MONTHLY MICROSCOPICAL JOURNAL. JUNE 1, 1870. I.—The New Binocular Microscope. By SAMUEL HOLMES. (Read before the ROYAL MICROSCOPICAL SOCIETY, May 11, 1870.) PLATE LIII. My microscopes are arranged to be Monocular, Binocular, Pseudoscopic, or Stereoscopic, at the pleasure of the observer. The definition and illumination are equal in both fields of view. They give pictures in solid relief of Opaque objects, and show the thickness of the structure in Transparent objects. There have been several forms of Binocular Microscope already contrived, but an examination of the principles of their construction shows that much has been left to future endeavour, In the binocular microscope of Nachet, since, I believe, somewhat modified, we have several extra surfaces of glass for the light to pass through in the necessary prisms, and irrespective of the difficulty of ensuring correct internal transmission through these prisms, their surfaces reflect and scatter the light that a more scientific combination might conserve. Mr. Wenham's first scheme, of two achromatic prisms over the object-glass, was open to the same objection, and had a similar defect of increased surface although in a less degree, and the difficulty of execution was great from the thinness and acuteness of the prisms. In Mr. Wenham's second plan, since so deservedly popular, the prism requires nice execution, and depends for the similarity of definition of its picture, on the equal transmission of light through a thick piece of glass, and on the goodness of its four surfaces. These difficulties, combined with the error acquired by the unequal length of one set of rays over that of the other set, render it almost impossible to make an instrument on this plan in which the errors are not perceivable by the eye; and thus, although the illusion of solidity is presented to the observer, a doubt has arisen as to whether it has been properly obtained and is to be trusted. The most perfect piece of glass is variable in structure, and affects the passage of light through it in an irregular manner. This may be called structural aberration, and is reduced to its least VOL. III. T effect in all optical instruments by making their glasses as thin as possible. Hence total-reflexion prisms, by their unavoidable thickness and the number of their surfaces, produce indistinctness that cannot be compensated or removed, and give to the pictures of an instrument in which they are used an unequal appearance and value, the blending of which, by two eyes, can never produce a correct stereoscopic relief. Largeness of aperture in the object-glass gives a certain relief to an object under view with one eye; this is simply monocular relief, and is untruthfully exaggerated by the two oblique reflexions in the prism, giving a lateral elongation to its image: thus the view of the left eye being rendered dissimilar, presents an appearance of relief when blended with the undistorted view of the right eye. The amount of this distortion may be seen in the diagram of the prism, &c., in which the two paths of the light to the right and left eye are produced backwards until they intersect the plane of the object. One path is vertical, the other oblique. Now the area of the oblique section is greater than that of the rectangular section, and if the area containing the object is distorted, the object is distorted also. Again, as the eye refers the position of an object to the direction from whence the light came, we may reasonably infer that the left eye sees an object considerably to the right of the real one, and the obliquity and lateral displacement have the effect of making a Cube appear as a Parallopipedon, and a Sphere as an Ellipsoid, and these forms superposed by any stereoscope will give a false perspective of the Cube and Sphere. From the above considerations, believing that the attempt to divert half the light from an objective to form a secondary image by a total-reflexion prism, to be incapable of improvement, I have experimented in other directions, and have even gained more light and more equality by the use of two prisms, having only a single reflexion in each. In January, 1869, I contributed a paper to the Quekett Club, on some experiments made in the year 1858, which was read by my co-member and friend Mr. George, and to which I now advert only as a matter of history. In the microscope therein described, I proposed to divide the light from the objective into three portions and direct one half into each eye, through the medium of two reflectors and eye-pieces. The object-glass shown to the meeting was a hemispherical plano-convex lens, and the plane side had two reflecting facets at the binocular angle. This lens was to be placed over the object with its angulated surface at an angle of 45° to the perpendicular, in which direction the rays from the object should reach it from below, to be divided and reflected horizontally to the eyes through two equally inclined eye-pieces. It was shown that, if the angulated surface was cut into the lens, the image was pseudoscopical; but that if the reflectors were made by cutting off the lens, the right eye would receive the left image, and vice versa, the result being a stereoscopic view. Now this contrivance, although optically qualified to give equal binocular pictures, had certain disadvantages that prevented me making further use of it, the greatest of these being, that the bodytubes must be horizontal, while the objective is vertical, which is rather unsightly and quite contrary to practice. The paper will be found at length in No. 9 of Journal of the Quekett Club, and was noticed in the Monthly Journal of this Society, No. III., March 1, 1869. The instrument I am now about to describe is on a different principle to the foregoing, and is my latest invention; my first view through the first one completed, dating 19th May, 1869, and I have since been employed in perfecting the mechanics of the design. Suppose, for illustration, we had a complete and well-appointed achromatic microscope, and we were to make a clean vertical section through the centre of the optical parts and their supporting brasswork. Now, although completely dissevered, we could see through it just as well as before its division. But the division of the eye-piece being near the eye would be visible, and therefore might be left undivided, as it is mainly with the object-glass that we have to deal. Now, being in possession of an imperceptibly bisected instrument, if we consider each half as the radius of a circle whose centre is the focus of the object-glass, we may, by separating the halves of the body at the upper end to the distance between the eyes and adding two eye-pieces, observe binocularly any object that was in focus when closed as a single body. And here we get an opportunity of proving the truth of the opinions of Harris, Goring, and Brewster, previously quoted. The binocular vision here provided is strictly natural, both eyes being used under exactly the same conditions as when examining an object unassisted at the distance of distinct natural vision, for the two converging optic axes are directed to the same object, and have the same assistance as to magnifying power and illumination, and the relief presented by an object of solidity must be absolute for quantity and correctness. The indirectness of the path of the light forming the second image in the Prism Binocular, before referred to, prevents the use of high powers, because the errors become too sensible to be disregarded. But in the bisected arrangement, the highest power attainable |