34 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



But he has further shown us 1 that with the best objectives of the 

 old construction, and with large apertures, the limits of a completely 

 satisfactory clearness of image are reached when the swper-amplifiea- 

 tion is four- to six-fold ; that is, when the total magnifying power of the 

 objective and eye-piece together is four to six times as great as that 

 obtained with the objective when used by itself as a magnifying 

 lens. On the other hand, with apochromatic objectives the available 

 super-amplification even with the greatest apertures is at least 

 twelve- to fifteen-fold, and considerably higher with medium and low 

 objectives. 



3. Achromatism touches almost an ideal point in these objectives. 

 The images are practically free from colour over the entire area. 

 This is of great value in photo-micography. The correction errors 

 of the ordinary achromatic systems are much more powerful as 

 disturbing influences than in ordinary observation with the eye. 



4. In spite of the removal of the secondary spectrum certain 

 colour deviations of a tertiary nature remained, and are inevitable 

 in all objectives of great aperture in which the front lens cannot be 

 made achromatic by itself. With ordinary achromatic objectives, 

 from the properties of the glass used, the amount of this is very un- 

 equal in the central and peripheral parts, but in the apochromatic 

 object-glass it is approximately constant for all parts of the opening, 

 and therefore it allows of correction by the eye-piece, a special con- 

 struction possessing equal but opposite differences of magnifying 

 power for different colours. The eye-piece is so constructed as to 

 completely secure the desired result, and, as we have stated above, 

 images free from colour are obtained. 



5. The classification of the eye-pieces for this system of objectives 

 has been established by Abbe, and depends on the increase in the 

 total magnifying power of the microscope obtained by means of the 

 eye-piece as compared with that given by the objective alone. The 

 number which denotes how many times an eye-piece increases the 

 magnifying power of the objective, when used with a given body- 

 tube, gives the proper measure of the eye -piece magnification, and 

 at the same time the figures for rational numeration. 2 



From their properties these are known as ' compensating eye- 

 pieces.' 



The following is a fair typical selection of the objectives and 

 eye-pieces furnished from the workshops of Carl Zeiss, of Jena, on 

 this important system, viz. : 



1 ' On the Relation of Aperture to Power,' Journ. B.M.S. 1883, p. 803. 



2 ' On Improvements of the Microscope with the aid of new kinds of optical glass ' 

 (Abbe), Journ. B.M.S. 1887, p. 25 et seq. 



