systems of wide aperture. On account of the slight depth definition, short focused systems of wide aperture are not at all specially suitable for stereoscopic observation, because the possibility of observing objects taking up a good deal of space is too limited when such systems are used.
Fig. 48.—Riddell’s Prisms.
Professor J. L. Riddell (Quart. Journ. Micros, 1853, p. 236; 1854, pp. 18–24) published an arrangement of prisms, which, however, imparted a pseudomorphous impression if image-forming oculars were not used, and in 1854 a second system (fig. 48), 
Fig. 49. Wenham’s Prism.essentially a Wheatstone pseudoscope, added just above the objective. This gave an orthoscopic image even in ordinary eyepieces. By adopting right-angled reflection-prisms above the eyepiece he completely erected the image. Stephenson’s stereoscopic microscope (fig. 58, Plate) resembles this apparatus in all essentials. A construction of prisms by Nachet is now almost forgotten, while on the contrary an extremely simple dividing prism published by Wenham (Lond. Micros. Soc., 1861, i. 109) has been exceptionally well attested in practice. It is more easily used than any other apparatus (see Binocular Instruments, fig. 8). A reflection-prism (fig. 49) in a setting is placed above the last surface of the objective and divides the exit rays. The group of rays coming from the left half of the objective can continue its way without hindrance to the right eye. The group of rays coming from the right half of the objective is reflected twice in the
prism and directed to the left eye. The tube containing the left eyepiece is a little inclined towards the right tube, which is perpendicular. It can be adapted to the interpupillary distance by changing the tube slide. If it is desired to use the instrument as a monocular, the setting with the prism at the lower end of the tube is taken away.
A second manner of making stereoscopic observations employs stereoscopic eyepieces. The first of such eyepieces was proposed by R. B. Tolles.[1] He realized that the division of the cones of rays by prisms could only be satisfactorily performed if the prism was placed in the position of the exit pupil of the objective or in the position of the real image of this exit pupil. He employed a Nachet combination of prisms and placed the dividing prism at the spot where a special reversing system formed a real image of the exit pupil of the objective. A second stereoscopic eyepiece was devised by A. Prazmowski who substituted a Wenham diffracting division prism at the position of the real image of the exit pupil of the objective formed by a reversing system. The newest form of a stereoscopic microscope resembles the oldest in so far as two completely separate microscopes. are used. In the oldest microscope by Cherubin d’Orleans the observer receives a pseudoscopic impression in consequence of the reversed image. This defect has been avoided in the instruments constructed in the Zeiss factory (fig. 59, Plate) at the instigation of the American zoologist H. S. Greenough. The system of Porro prisms employed affords a convenient method of adapting the ends of the eyepieces to the interpupillary distance. The two tubes are inclined to one another at an angle of about 14°. The microscope is only intended for slight magnifications. The possibility already suggested of using both eyes for observing without having a stereoscopic impression, is often regarded as a great advantage. Binocular microscopes have therefore been constructed on this plan. Such a combination of prisms was used by Wenham, who placed it directly behind the last objective lens. As a rule this arrangement of prisms can be exchanged for the Wenham stereoscopic reflection-prisms.
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| Fig. 50. Powell’s Prisms. |
A second kind of dividing prism which directs the entire course of rays to both eyes, and thus produces identical images, was used by Powell and Lealand (fig. 50). Every ray is divided into a reflected and a refracted portion on the front side of a parallel plate. Whilst the refracted portion after leaving the plate continues its way in the same direction, displaced a little to one side, the reflected portion is directed into the side tube by a reflection-prism. With these microscopes, which are not stereoscopic, objectives of any power can be used. The surfaces of the dividing prisms must be very exact, so that no deterioration of the image may arise from them. A microscope for two eyes can also be by employing the Abbe stereoscopic eyepiece.
By the supplementary use of one of Wenham’s prisms every ray is analysed into a more powerful refracted and a weaker reflected one. The same image can be presented to each eye by using this eyepiece also. No stereoscopic impression is then felt. It is brought about by placing special semicircular diaphragms in the plane of the exit pupil of the microscope. By turning the diaphragms 180° round the optical axis, the orthoscopic impression can be changed into the pseudoscopic. The mechanical arrangement of the eyepiece is such that the distance of the two exit pupils can be adjusted to the interpupillary distance.
Mechanical Arrangements
Although the optical. system is the first consideration in a microscope, the system is valueless if the fittings do not allow its correct use. The optical system must be kept at a certain distance and well centred, and a correct position for the object in relation to the system must be assured.
In fig. 60, Plate, the microscope is seen to consist of the heavy metal foot A, which rests on the table at three points. The whole microscope is fitted to this foot. The object can be held firmly on the stage plate B by cramps C. On the lower side of the stage plate are the condenser and the diaphragms, and the illuminating mirror I is held by a rod D fixed to the stage plate. Likewise on the stage plate is the support for the tube E. The rough adjustment of the microscope can be made by a rack and pinion F; and the fine adjustment by the screw G. The tube containing the eyepiece and the objective is double. The inner tube H is movable, making a change in the length of the tube possible. As a rule this inner tube has a mark which allows the length of the tube to be set.
It is most important the stand should be free of vibration. A fine adjustment is also necessary, in order to perform conveniently and with certainty the slight motion of the microscope in relation to the object. In cheap stands the rough adjustment was worked by moving the inner tube by hand, but the more convenient rack and pinion is now used almost exclusively.
For slight magnifications rough adjustment is sufficient, but with objectives of a focus below 14 in., a fine adjustment is wanted. Very different constructions are in use. Almost all are such that the whole microscope tube is raised or sunk by the mechanism of the fine adjustment, and not only the objective. The most used is the micrometer screw adjustment (fig. 51).
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| Fig. 51.—Micrometer Screw Adjustment of Leitz. |
Fig. 52.—Lever Adjustment of J. Swift & Son. |
- ↑ R. B. Tolles, Sill. Journ. (1865). xxxix. 212; Journ. Roy. Micr. Soc. (1890), pt. i. p. 383.
