(E) BINOCULAR MICROSCOPES (cont.)
Non-Stereoscopic Binoculars.—The great comfort which is experienced by the microscopist in the conjoint use of both eyes has led to the invention of more than one arrangement by which this can be secured when those high powers are required which cannot be employed with the ordinary stereoscopic binocular. This is accomplished by Messrs Powell and Lea-land by taking advantage of the fact that, when a pencil of rays falls obliquely upon the surface of a refracting medium, a part of it is reflected without entering that medium at all. In the place usually occupied by the Wenham, prism they interpose an inclined plate of glass with parallel sides, through which one portion of the rays proceeding upwards from the whole aperture of the objective passes into the principal body with very little change in its course, whilst another portion is reflected from its surface into a rectangular prism so placed as to direct it obliquely upwards into the secondary body (fig. 26). Although there is a decided difference in brightness between the two images, that formed by the reflected rays being the fainter, yet there is marvellously little loss of definition in either, even when the inch objective is used. The disk and prism are fixed in a short tube, which can be readily substituted in any ordinary binocular microscope for the one containing the Wenham prism.
Other arrangements were devised long ago by Air Wenham, [Footnote 274-5] with a view to obtain a greater equality in the amount of light-rays forming the two pictures; and he has latterly carried one of these into practical effect, with the advantage that the compound prism of which it consists has so nearly the same shape and size as his ordinary stereoscopic prism as to be capable of being mounted in precisely the same manner, so that the one may be readily exchanged for the other. The axial ray a, proceeding upwards from the objective, enters the prism ABDEF (fig. 27) at right angles to its lower face, and passes on to c, where it meets the inclined face AB, at which this prism is nearly in contact with the oblique face of the right-angled prism ABC. By internal reflexion from the former and external reflexion from the latter about half the beam b is reflected within the first prism in the direction cb, while the other half proceeds straight onwards through the second prism, in the direction ca', so as to pass into the principal body. The reflected half, meeting at d the oblique (silvered) surface DE of the first prism, is again reflected in the direction db', and, passing out of that prism perpendicularly to its surface AF, proceeds towards the secondary body. The two prisms must not be in absolute contact along the plane AB, since, if they were, Newton's rings would be formed; and much nicety is required in their adjustment, so that the two reflexions may be combined without any blurring of the image in the secondary body.
For the prolonged observation, under high powers, of objects not requiring the extreme of perfection in definition,—such, for example, as the study of the cyclosis in plants,—great advantage is gained from the conjoint use of both eyes by one of the above arrangements.
Footnote
274-5 Transactions of the Micros. Soc., N. S., vol. xiv., 866. p. 105.
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