tions the microscope illustrated in Fig. 3 is recommended, as it is at once simple in construction and reliable in optical performance. A good microscope, as has been said should consist of the following parts: the stand; the body, with coarse and fine adjustments; the stage, simple or mechanical; the substage, with Abbe condenser; the mirrors, plane and concave, and objectives and eye-pieces. The Stand.-The first requisite in the stand is perfect steadiness, as it has to support the weight of the body and its attachments, as well as the pressure brought to bear upon it during work. To secure steadiness lengths in different instruments and this is of great importance in selecting objectives, as good objectives are always corrected for a certain known tube length. There are two standards for tube length, the English and the Continental. The English mechanical tube length is 834 in., while the Continental mechanical tube length is 6.3 in. The latter is, in the author's opinion, most satisfactory, as it is easier to work with. It should be remembered that an objective corrected for a Continental tube length will not give its best results if used in an English standard tube. Coarse and Fine Adjustments. two forms of base are chiefly used, i. e., the horseshoe and the tripod. In the horseshoe form the result is obtained by weight and the addition in the microscopes mentioned above of a projecting arm at the centre of the bend of the horseshoe. This form of base is most satisfactory and secures perfect steadiness. The stand is provided with an arm holding the tube, and this arm should always be capable of inclination to a horizontal position, if desired. The Body.-The body, or tube for receiving the eye-pieces and objectives, consists of a metal tube containing at its upper end a shorter tube, known as the draw-tube. The tube or body may be of various Every good microscope has two methods for securing the correct focus, i. e., the coarse and the fine adjustment. The coarse adjustment in the older forms of microscopes was secured by means of a draw-tube playing within the body, but it was at its best clumsy and unreliable. The coarse adjustment should always be by rack-and-pinion, and in no other way. The diagonal rack-and-pinion used in microscopes mentioned above is in every way excellent, as the surfaces on the tube are made on a separate piece of metal, which is attached to the tube, and the pinion box is provided with a tension for taking up wear. A good coarse adjustment should move smoothly and easily, and be capable of clearly focusing low powers and also, fairly well, high powers. The fine adjustment is used in all work requiring high power, especially in bacteriological work, and it is of the utmost importance that it should be perfect in construction and performance. It is unnecessary here to enter into a discussion of the various forms of fine adjustment used, as they all, in a greater or less degree answer the purpose for which they are constructed, but the fine adjustment recently adopted by gular in shape, and of metal or hard rubber, having smooth surfaces and perfect rigidity. For ordinary work the metal stage covered with hard rubber is preferable as such a stage is not affected by such reagents as may be used. An iris diaphragm, arranged so as to be used either with or without a condenser, and placed in the plane of the stage, is an improvement recently introduced and one which will be appreciated by every microscopist. Such stages are fitted to the microscopes illustrated in Figs. 2 and 3. A mechanical stage is a necessary Bausch and Lomb in their Continen- part of every first-class microscope. tal models is worthy of special mention. In this form the micrometer screw acts directly upon the triangular bearing of the arm, while the weight of the body is balanced by a spring, thus relieving the screw of all but the minimum strain. Lateral motion is impossible and the adjustment is very sensitive and accurate. In using the fine adjustment it should never be turned rapidly, or rolled in the fingers, but always slowly and carefully. The Stage. The stage should be large and either circular or rectan It should be fitted with rack-and-pinion adjustments, and have equal speed in vertical and horizontal movements. Such a stage is of great value in bacteriological work, and in examinations of urine and sputum, as by it every portion of the slide can be carefully gone over. The mechanical stage illustrated in Fig. 4 is the one used by the author and is perfectly satisfactory in every respect. The horizontal and vertical movements obtained by rack-and-pinion, of equal speed, while millimeter graduations with verniers are attach ed to both movements. By this means an object once found may be always refound by simply noting in the catalogue or upon the slide, the position occupied upon the vertical and horizontal scales. The stage rotates upon its axis, and the circumference is divided into 360 degrees, the vernier reading to tenths of a degree. It is provided with centering screws and can be easily removed from the microscope when a plane stage is desired, as in examination of cultures, etc. The microscope illustrated in Fig. 1 is provided with this stage. iris diaphragm, which is so arranged that it is in contact with the slide, where it is most useful if the condenser is not used. The middle portion of the substage is movable vertically upon the substage axis, and consists of a ring supporting the Abbe condenser, 1.20 N. A. The condenser can be swung laterally, so as to be out of the way if necessary. The Abbe condenser is of the utmost importance in clinical work, especially in bacteriological examinations. Without it the microscope, considered optically, is an abortion, while with it, it becomes truly a scientific instru The Substage and Condenser.-Almost as important as the objectives is a good substage and condenser. The substage is intended to hold the condenser and other apparatus necessary to properly illuminate the ob ject to be examined, and upon its performance depends the result of some of the finest work done in microscopy. The complete substage attachment, as illustrated in Figs. 5 and 6, may be said to consist of three parts, an upper, middle and lower portion. The upper part is a fixed metal ring which contains the removable ment. The condenser consists of a series of lenses so arranged as to concentrate the rays of light fully upon the object to be examined and its performance in this respect is limited by the iris diaphragm, which is placed behind it and controls the amount of light entering the condenser. The third portion of the substage holds the large iris diaphragm, which is used below the condenser. This portion of the substage may be swung laterally to the right, as is shown in Fig. 5, and is laterally movable by rack-and-pinion when it is desired to use oblique illumination. The objectives should be made by a reliable optician and carefully corrected and examined before they are accepted, as upon them depends chiefly the value of the instrument. Quality should not be sacrificed to cheapness in the purchase of any part of the microscope and never in the selection of the objectives. The movement of the entire sub-in. stage when closed, as shown in Fig. 6, is by rack-and-pinion, which is attached to a slide resting upon a heavy metal bar, which forms a part of the main stand. The entire substage may be removed by simply sliding it off this metal bar. This substage is supplied with the microscopes illustrated in Figs. 1 and 2. The Mirror.-The mirror of the microscope should be from two and a half to three inches in diameter, plane upon one side and concave upon the other. It is used to focus the light upon the object to be examined, Triple Nose-Piece.-The triple nose. piece illustrated in Fig. 7, is a great convenience in microscopical work, as it holds all the objectives necessary, and they can be used as needed without the bother of attaching them each time to the body tube. and particles of foreign matter. The plane mirror should always be used with the condenser. Oculars and Objectives.-The ocular or eye-piece should be of the Continental types, when used upon a Continental stand, and two, a 1 in. and 11⁄2 in. should be procured if possible. If only one is selected, the 11⁄2 in. will be found most serviceable. The clinical microscope should be provided with at least three objectives, i. e., a 1 or in., a torin., and a in. oil immersion, the preference being given to the 1 in., and in., and Fig. 7. Triple Nose-piece, for objectives. Accessory Apparatus. Micrometer Eye-Piece.-In bacteriological work it is often necessary to make measurements of the bacteria and for this purpose various forms of micrometers have been devised. The |