Although the blue and the red rays of certain selected wave lengths can be brought to a common focus in this way, rays of other wave-lengths will not come to the same focus as the red and blue rays, owing to the differences between the relative "partial" dispersions of the two glasses. The lens thus gives an image which is slightly coloured at the edges, and is said to show "secondary colour." In modern apochromatic lenses three selected wave lengths are made to have a common focus; thus only a trace of "tertiary colour" is present and this in some instances is so small as to be unnoticeable. It has not been found possible, as yet, to design apochromatic lenses which give precisely equal magnifica tion for light of different colours. This can be corrected by a suit able eyepiece called a "compensating eyepiece" (see p. 438). An achromatic lens is usually corrected spherically for one colour only, an apochromatic lens should be corrected so as to have no spherical aberration for light of two different colours.
Other Aberrations of Object-glasses are :--curvature of field, distortion, astigmatism, and extra-axial coma. These are aberra tions of oblique pencils; their effect is to impair the image of any object-details lying off the axis, and to cause the image to lie on a curved surface so that it cannot all be seen in focus at one time. These defects are not of great importance in the ordinary use of the microscope since, if the sine-condition is satis fied, the definition is good over an appreciable area round the centre of the field where the field is flattest. For microphoto graphic work, however, these aberrations are most objectionable. It is possible to select types of design such that, in correcting the lens for axial, spherical and chromatic aberrations, the aberrations of oblique pencils are kept within small limits over a fairly large angle. Any object-detail to which particular attention is being paid should be brought into the middle of the field of view, i.e., on to the axis of the object-glass.
Monochromatic Object-glasses for Visual Work.—For the pur pose of obtaining higher resolving powers than have hitherto been obtainable with achromatic and apochromatic object-glasses, cer tain monochromatic lenses have been made, for use with radia tions of one wave-length only. This method makes it possible to obtain full correction for spherical aberration over a numerical aperture larger than that over which combined chromatic and spherical correction has hitherto been satisfactorily accomplished. Modern apochromatic immersion lenses of high quality are made with numerical apertures up to 1.4, while monochromatic lenses, fully corrected up to 1.6, have been designed.
object will then be formed at a particular distance behind the lens and, if the lens is properly designed and made, this image will be free from spherical aberration. If the cover-glass is not of the proper thick ness the image will not be free from spher ical aberration and, to correct the aberra tion thus introduced, the distance between the object and the object-glass must be altered. This will alter the distance be tween the image and the object-glass, so the length of the microscope tube (the dis tance between the object-glass and the eye piece) will have to be altered accordingly.
The type of aberration introduced is best explained in terms of the aberration of an ordinary uncorrected lens. A system in which the outer rays intersect the axis at a point nearer the lens than the intersec tion of the central rays is an "under-corrected" system. An "over corrected" system has a shorter focal length for the central rays than for the outer rays (fig. 16).
The effect of increasing the thickness of the cover-glass is indi cated in fig. 17, as is also the way in which a change in the tube length can be made to correct the aberration thereby intro duced. The lens is supposed to be corrected for use with an un covered object when the image of this object is formed at a dis tance L behind the lens (fig. 17.a). A lens corrected for use with a particular tube-length when used on uncovered objects, can be satisfactorily corrected for covered objects by shortening the tube length appropriately. Conversely, a lens corrected for covered objects can be used satisfactorily for uncovered objects if the tube-length is appropriately increased.
For use in microscopes with fixed tube-lengths, Amici made separate object-glasses to suit different thicknesses of cover-glass, rib. INTRODUCED UT A COVER-GLA55 (a) Lens fully-corrected for use with uncovered object when image is formed at distance L. (b) Same lens, with object under cover-glass and focus of central pencils at distance L. Ray system is under-corrected on object side and over-corrected on image side. (c) Same lens, with object as before. By increasing the distance of the lens from the object, the distance of the image is reduced to and full correction on the image side is obtained modifying the corrections but preserving the same aperture and power. This was made unnecessary by the introduction in 1837, by Andrew Ross, of object-glasses furnished with a "correc tion-collar" which enabled the spherical aberration of the object glass to be adjusted so as to compensate for the aberrations pro duced by cover-glasses of different thicknesses. The correction collar was improved later by H. F. Wenham, of the firm of Ross, and still later by Zeiss (fig. 18).