It very early became obvious to those who were engaged in the achromatization of microscopic objectives that their best performance was obtained when the image given by them was further enlarged by the eye-piece known as the Huygenian, as having been devised by Huygens for his telescopes. It consists of two plano-convex lenses (EE and FF, fig. 4), with their plane sides towards the eye; these are placed at a distance equal to half the sum of their focal lengths,or, to speak with more precision, at half the sum of the focal length of the eye-glass, and of the distance from the field-glass at which an linage of the object-glass would be formed by it. A "stop" or diaphragm BB must be placed between the two lenses, in le visual focus of the eye-glass, which is, of course, the position wherein the image of the object will be formed by the rays brought into convergence by their passage through the field-glass. Huygens devised this arrangement merely to diminish the spherical aberration; but it was subsequently shown by Boscovich that the chromatic dispersion was also in great part corrected by it. Since the introduction of achromatic object-glasses for compound microscopes, it has been further shown that nearly all error may be avoided by a slight over-correction of these, so that the blue and red rays may be caused to enter the eye in a parallel direction (though not actually coincident), and thus to produce a colourless image. Thus let N, M, N (fig. 14) represent the two extreme rays of three pencils, which without the field-glass would form a blue image convex to the eye-glass at BB, and a red one at RR; then, by the intervention of the field-glass, a blue image concave to the eye-glass is formed at B'B', and a red one at RR'. As the focus of the eye-glass is shorter for blue rays than for red rays by just the difference in the place of these images, their rays, after refraction by it, enter the eye in a parallel direction, and produce a picture free from false colour. If the object-glas had been rendered perfectly achromatic, the blue rays, after passing through the field-glass, would have been brought to a focus at b, and the red at r; so that an error would be produced, which would have been increased instead of being corrected by the eye-glass. Another advantage of a well-constructed Huygenian eye-piece is that the image produced by the meeting of the rays after passing through the field-glass is by it rendered concave towards the eye-glass instead of convex, so that every part of it may be in focus at the same time, and the field of view thereby rendered flat. [Footnote 267-1]
Two or more Huygenian eye-pieces, of different magnifying powers, known as A,B,C, &c., are usually supplied with a compound microscope. The utility of the higher powers will mainly depend upon the excellence of the objectives; for, when an achromatic combination of small aperture which is sufficiently well corrected to perform very tolerably with a "low" or "shallow" eye-piece is used with an eye-piece of higher magnifying power (commonly spoken of as a "deeper" one), the image may lose more in brightness and in definition than is gained by its amplification, while the image given by an objective of large angular aperture and very perfect correction shall sustain so little loss of light or of definition by "deep eye-piecing" that the increase of inagnifying power shall be almost clear gain. Hence the modes in which different objectives of the same power, whose performance with shallow eye-pieces is nearly the same, are respectively affected by deep eye-pieces afford a good test of their respective merits, since any defect in the correc-tions is sure to be brought out by the higher amplification of the image, while a deficiency of aperture is manifested by the want of light. The working microscopist will generally find the A eye- piece most suitable, B being occasionally employed when a greater power is required to separate details, whilst C and others still deeper are useful for the purpose of testing the goodness of objectives, or for special investigations requiring the highest amplification with objectives of the finest quality. But he can commit no greater error than habitually to use deep eye-pieces for the purposes of scientific research, especially when (as in the study of living objects) long-continued and unintermitted observation is necessary. For the visual strain thus occasioned is exactly like that resulting from the habitual use of magnifying spectacles in reading, requir-ing the book to be held within 2 or 3 inches of the eye. And all experience shows that this feeling of strain cannot be dis-regarded, without the most injurious consequences to vision.
For viewing large flat objects, such as transverse sections of wood or of Echinusa spines, under low magnifying powers, the eye-pieces known as Kellners may be employed with advantage. In this, construction the field-glass, which is a double-convex lens, is placed in the focus of the eye-glass, without the interposition of a diaphragm; and the eye-glass is an achromatic combination of a plano-coneave of flint with a double-convex of crown, which is slightly under- corrected, so as to neutralize the over-correction given to the objectives for use with Huygenian eye-pieces. A flat well-illuminated field of as much as 14 inches in diameter may thus be obtained with very little loss of light; but, on the other hand, there is a certain impairment of defining power, which renders the Kellner eye-piece unsuitable for objects presenting minute structural details; and it is an additional objection that the smallest speck or smear upon the surface of the field-glass is made so unpleasantly obvious tbat the most careful cleansing of it hat surface is required every time that this eye-piece is used.
Hence it is better fitted for the occasional display of objects of the character already specified than for the ordinary wants of the working microscopist.
Solid eye-pieces, consisting of cylinders of glass with convex ends, are, sometimes used in place of the Huygenian, when high magnifying power is require for testing the perforniance of objectives. The lower surface, which has the lesser convexity, serves as a field-glass; while the image formed by this is magnified by the highly convex upper surface to which the eye is applied,the advantage derivable from this construction lying in the abolition of the, plane surfaces of the two lenses of the ordinary eye-piece. [Footnote 267-2]
A "positive" or Ramsdens eye-piecein which the field-glass, whose convex side is turned upwards, is placed so much nearer the eye-glass that the image formed by the objective lies below instead of above itwas formerly used for the purpose of micrometry,divided glass being fitted in the exact plane occupied by the image, §o that its scale and that image are both magnified together by the lenses interposed between them and the eye. The same end, however, may be so readily attained with the Huygenian eye-piece that no essential advantage is gained by the use of that of Ramsden, the field of which is distinct only in its centre.
267-1 The reader may be referred to Air Varleys investigation of the properties of the Huygenian eye-picee in the fifty-first volume of the Transactions of the Society of Arts; and to the article "Microscope," by Mr Ross, in the Penny Cyclopaedia, reprinted, with additions, in English Cyclopaedia.
267-2 These eye-pieces are much in vogue in the United States, where they are made of extremely short focieven to 1/16 inch.
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