Fig. 99.—Huyghenian Eye-piece A, the dotted lines show position of lenses.
It was reserved for Boscovich to point out that, by this important arrangement, he had corrected a portion of the chromatic aberration incidental to the earlier form of eye-pieces. Let [Fig. 100] represent the Huyghenian eye-piece of a microscope, f f being the field-glass, and e e the eye-glass, and l m n the two extreme rays of each of the three pencils emanating from the centre and ends of the object, of which, but for the field-glass, a series of coloured images would be formed from r r to b b; those near r r being red, those near b b blue, and the intermediate ones green, yellow, and so on, corresponding with the colours of the prismatic Spectrum.
The effect described, that of projecting the blue image beyond the red, over-correcting the object-glass as to colour, is purposely produced; it is also seen that the images b b and r r are curved in the wrong direction to be seen distinctly by the convex eye-lens; this then is a further defect of the compound microscope made up of two lenses. But the field-glass, at the same time that it bends the rays and converges them to foci at b′ b′ and r′ r′, also reverses the curvature of the images as here shown, giving them the form best adapted for distinct vision by the eye-glass e e. The field-glass has at the same time brought the blue and red images closer together, so that they produce an almost colourless image to the eye. The chromatic aberration of lenses has been clearly explained in a previous chapter. But let it be supposed that the object-glass had not been over-corrected, that it had been perfectly achromatic; the rays would then have appeared coloured as soon as they had passed the field-glass; the blue rays of the central pencil, for example, would converge at b′′, and the red rays at r′′, which is just the reverse of what is required of the eye-lens; for as its blue focus is also shorter than its red, it would require that the blue image should be at r′′, and the red at b′′. This effect is due to over-correction of the object-glass, which removes the blue foci b b as much beyond the red foci r r as the sum of the distances between the red and the blue foci of the field-lens and eye-lens; so that the separation b r is exactly taken up in passing through those two lenses, and the several colours coincide, so far as focal distance is concerned, as the rays pass the eye-lens. So that while they coincide as to distance, they differ in another respect—the blue image is rendered smaller than the red by the greater refractive power of the field-glass upon the former. In tracing the pencil l, for instance, it will be noticed that, after passing the field-glass, two sets of lines are drawn, one whole and one dotted, the former representing the red, and the latter the blue rays. This accidental effect in the Huyghenian eye-piece was pointed out by Boscovich. The separation into colours of the field-glass is like the over-correction of the object-glass—and opens the way to its complete correction. If the differently-coloured rays were kept together till they reached the eye-glass, they would still be coloured, and present coloured images to the eye. The separating effected by the field-glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to its spherical figure, the refractive power is less than at the margin, so that spherical error of the eye-lens may be said to constitute a nearly equal balance to the chromatic dispersion of the field-lens, and the blue and red rays l′ and l′′ emerge nearly parallel, presenting a fairly good definition of a single point to the eye. The same may be said of the intermediate colours of the other pencils. The eye-glass thus constructed not only brings together the images b′ b′, r′ r′, but it likewise has the most important effect of rendering them flatter, and assisting in the correction of chromatic and spherical aberration.
Fig. 100.—Huyghenian Eye-piece.
Fig. 101.—Ramsden’s Eye-piece.
The later form of the Huyghenian eye-piece is that of the late Sir George Airy, the field-glass of which is a meniscus with the convex side turned towards the objective, and the eye-lens a crossed convex with its flatter side towards the eye. Another negative eye-piece is that known as the Kellner, or orthoscopic eye-piece. It consists of a bi-convex field-glass and an achromatic doublet eye-lens. This magnifies ten times, but it in no way compares with the Huyghenian in value. Neither does it afford the same flatness of field.
The Ramsden, or positive eye-piece, is chiefly employed as a micrometer eye-piece for the measurement of the values of magnified images. The construction of this eye-piece is shown in [Fig. 101], a divided scale being cut on a strip of glass in 1⁄100ths of an inch, every fifth of which is cut longer than the rest to facilitate the reading of the markings, and at the same time that of the image of the object, both being distinctly seen together, as in the accompanying reduced micro-photograph of blood corpuscles, [Fig. 102].