MICROSCOPE LENSES The manufacture and of microscope lenses involves peculiar difficulties, from the smallness of the lenses and their short radii of curvature. The lens of a microscope is always either hemispherical or more than hemi spherical in shape, and often less than in. diameter. When it is borne in mind that the radius of curvature of these small lenses to be made to the calculated radius to within in. the necessity of special means to ensure these conditions will be realized.
pitch is employed as a polisher it is more customary to use a mixture such as shellac and putty powder, and the meth ods of and already described are then applicable with such modifications as are implied by the above mentioned characteristics of the deeper lenses used for microscope objectives. Very accurate means of the radius of curvature have to be employed. One method in use employs two microscopes fixed so that their axes make a with one another. The lens surface to be tested is placed on a table in such a way that it can reflect the back into one microscope or the other alternatively, to the position occupied by the table. The distance traversed taken with the separation of the axes of the microscopes enables the radius of curvature of the lens surface to be calculated.
As already stated nearly all the front lenses of power objectives are hemispherical or more than hemispherical, and to determine the departure from sphericity of such a lens is one of the difficulties of the microscope manufacturer. The method of does not differ in principle from that adopted in the case of lenses. The successful and of the lens is, however, an operation of considerable skill. The mechanical accuracy of the mount also special care, as does the process of and the front lens, the spherical surface of which bears a shoulder, the lens centred and cemented in position with shellac.
power is to be found in the Grayson of ten fine diamond lines ruled on surfaces and separated by distances down to in. The manufacturer, however, re to assist him in perfection in his lenses an indi cation not only that the lens is imperfect, but of the nature of its imperfection. The reflection of from a very small mercury him a small point of which will serve a purpose similar to that of the star in the observation of the diffraction rings in the testing of telescope lenses. In place of the mercury globule, the light transmitted through a selected accidental per foration in a piece of silvered glass can be used with advantage. The observation of the ring system inside and outside focus with either of these kinds of object affords in either case an indication of the accuracy of the aberration correction or of the manufacture of the lens. Here again, however, a special difficulty arises in the case of the microscope, for the aberrations which in the case of a telescope are simple (involving in the calculations terms of the second order only), in the case of the microscope are much more complex; this complexity is reflected in the appearance of the rings, not only the brightness but their disposition also being strongly affected by the aberrations of the various zones. Thus judgment of the correction required is very difficult. Information of a more direct character is obtained from observing the appear ance within and without focus on the exposure one by one of narrow zones of the objective, but here again considerable expe rience is needed before a right judgment can be come to. It should here be noted that even in the repetition manufacture of an ap proved type of objective the maker requires to make a final cor rection of aberration, which may be effected by slightly altering the position of one of the lenses. The means that appears likely to afford the most direct information on this subject is provided by the microscope objective interferometer, the application of which to this problem of the aberrations of microscope lenses seems likely to afford valuable information. (For aberration in optical systems, see OPTICS.) (F. T.)