Page:EB1911 - Volume 18.djvu/423

 noticeable. For this reason Amici constructed objectives of a similar aperture and focus for different thicknesses of glass covers.

This expensive method was simplified in 1837 by Andrew Ross by making the upper and lower portion of the objective variable by means of a so-called correction-collar, and so giving the objective a corresponding under-correction according to the thickness of the glass cover. The alteration of the focus and the aperture are little influenced. The correction-collar was improved by Wenham and Zeiss, by working the upper system upon the lower, and not the reverse; for in this way the preparation remains almost exactly focused during the operation (see fig. 34). The injurious influence of the glass cover is substantially lessened if no air is admitted to the space between the glass cover and the lens (as in the dry-system) but if the intervening space is filled with an immersion-liquid. Amici was likewise the first to produce practical and good immersion-systems. The slight difference of the refractive indexes of the glass cover and the immersion-liquid involves a diminution of the aberrations, by which the objective will become less sensitive to the differences in thickness of the glass covers and admits of a more perfect adjustment. Water-immersion was introduced by Amici in 1840, and was improved by E. Hartnack in 1855.

Thi advantages of the immersion over the dry-systems are greatest when the embedding-liquid, the glass cover, the immersion-liquid and the front lens have the same refractive index. Such systems with a so-called homogeneous immersion were first constructed after the plan of E. Abbe in 1878 in the Zeiss workshops at the instigation of J. W. Stephenson. Cedarwood oil (Canada balsam), which has a refractive index of 1·515, is the immersion-liquid. The structure of a modern system of this type, with a numerical aperture of 1·30, is shown in fig. 35.

The most perfect microscope objective was invented by E. Abbe in 1886 in the so-called apochromatic objective. In this, the secondary spectrum is so much lessened that for all practical purposes it is unnoticeable. In the apochromats the chromatic difference of the spherical aberrations is eliminated, for the spherical aberration is completely avoided for three colours. Since in these systems the sine-condition can be fulfilled for several colours, the quality of the images of points beyond the axis is better. There still remains a slight chromatic difference in magnification, for although the magnification consequent upon the fulfilment of the sine-condition is the same for all zones for one colour, it is impossible to avoid a change of the magnification with the colour. Abbe overcame this defect by using the so-called compensation ocular, made with Jena glasses. Fig. 36 shows, an apochromat of a numerical aperture of 1·40.

The eyepiece is considerably simpler in its construction than the objective.

Its purpose in a microscope is by means of narrow cones of rays to represent at infinity the real magnified image which the objective produces. As, however, the object represents a real image, the problem is to project a transparent diapositive. It is therefore impossible to observe this image through an ordinary lens. Since many of the rays coming from the exit-pupil of the objective would not reach the eye of the observer at all, it is necessary, in order to make use of all of them, to direct the diverging rays forming the real image so that the whole of the light enters the eye of the observer. This is effected by a collective lens; it may be compared with the second part of the condenser system of a projecting lantern.

The two most customary eyepieces consist in two simple plano-convex lenses, whose distance one from the other is equal to half the sum of the two focal lengths. One of these is the Ramsden eyepiece (fig. 37). If the real image produced by the objective coincides with the collective lens, only the inclination of the principal rays is altered, the form of the cone being affected only to a very small extent. The lens nearer the eye, which has about the same focal length as the collective lens, is distant from it by about its focal length. The eye-lens converts diverging pencils into parallels. Both lenses together form the exit-pupil of the objective behind the eye-lens, so that this image, the exit-pupil of the total system or the Ramsden circle, is accessible to the eye of the observer. It is possible to see the whole field through this pupil by slightly moving the head and eye. In practice the real image is formed not directly on the collective lens but a little in front of it, because otherwise all the particles of dust on the collective would also be seen magnified. In the other type, the Huygenian eyepiece (fig. 38), which is much more widely used, the collective lens is in front of the real image; it alters the direction of the principal rays and somewhat diminishes the real image. In this type the eye-lens is about twice as powerful as the collective lens, and makes the rays parallel. Here also the exit-pupil is accessible to the eye and through it the whole field can be seen by moving the head and eye. In both eyepieces micrometers or cross-wires are used for measuring in the plane of the real image. The Ramsden eyepiece is the most convenient for this because this plane lies in front of the collective lens, and the objective image has not yet been influenced by the eyepiece.

As both eyepieces are used with very small apertures (about f&#x202f;:20) no attempt has been made to overcome the spherical aberrations, which are usually very slight; neither, as a rule, are the eyepieces chromatically corrected, care has only to be taken by a suitable choice of the distance of one lens from the other, that the coloured images derived from a colourless object should have the same apparent size. Since, however, the difference of chromatic magnification cannot be overcome in powerful objectives, this error is still further increased by the eyepiece. The difference of chromatic magnification cannot even be overcome in apochromats, and to cancel this aberration Abbe devised the compensating ocular (fig. 39). The weak compensation oculars resemble a Huygenian eyepiece with achromatic eye-lens, whilst the more powerful ones are of a different construction. These eye-pieces are intentionally provided with a different chromatic magnification, which however is in opposition to that originating in the objective. They have also a shorter focus for red, and a longer one for blue, and thus magnify the red image more than the blue; and as the objective gives a large blue and a small red image, the two cancel one another and a colourless image is produced.

These eyepieces are very convenient in use, for when they are changed the lower focus always falls in about the same plane. In German and French microscopes the optical length of the tube, when apochromats and compensation-eyepieces are used, is 180 mm. By multiplying the magnification of the objective by the number