On Improvements of the Microscope. By Prof. E. Ahhe. 25 



in regard to definition as the impression which is produced upon the 

 retina by the latter. 



According to theory, the photographic depiction of the microscopic 

 image ought to have a not inconsiderable advantage as compared with 

 eye-piece observation, on account of the essentially shorter wave-length 

 of the light which is employed in the former case ; with photography 

 the objective produces an effect equal to an increase of aperture in the 

 ratio of about 4 : 3 as compared with eye-piece observation. If, hitherto, 

 as it appears, such an advantage has not been apparent in practice, the 

 cause must be, in accordance with what has been said above, that the 

 images formed by the chemical rays have never been so perfect, as 

 regards concentration, as the visible images, and that this defect has 

 counterbalanced the advantage of shorter wave-length. Experiments 

 which have since been made with some of the objectives, constructed 

 on the new principles, give ground for the expectation that in future 

 the theoretical superiority of the photographic method will be realized. 



The above-mentioned aims for the improvement of objectives, as well 

 as indications of the way in which they are to be practically secured, 

 were stated by the author some years ago.* He has also described ex- 

 perimental objectives which, with the use of strongly refracting Hquids 

 in the form of inclosure-lenses were made as early as 1873 in Zeiss's 

 workshops,! with the object of practically testing his views, and, in the 

 words which he then used, of getting " a glance at the Microscope of the 

 future." So far therefore the present objectives are only the final 

 elaboration of a plan for the improvement of the Microscope which 

 has been pursued in these workshops for many years, but which remained 

 so long in suspense because the glass manufacture could not supply 

 the requisite material. 



The elaboration of this plan has moreover led to a fm*ther advantage 

 which, though of secondary importance, yet appears to be a desirable 

 improvement. In all objectives of large aperture, in which the front- 

 lens cannot be made achromatic by itself, there remains, even when 

 the colour deviation along the axis (i. e. in the centre of the field) has 

 been corrected as completely as possible, a not inconsiderable difierence 

 in the magnifying power for different colours (difference of the focal 

 length of the objective for different colours when the position of the 

 anterior focal point is the same), and this gives rise to marked colour 

 deviation outside the centre of the field which makes itself apparent in 

 conspicuous borders of colours at the margin. (The image formed by 

 the blue and violet rays is larger than that of the red and yeUow, it 

 coincides with the latter at the centre of the field but extends over it 

 more and more towards the margin.) 



This defect of amplification cannot be corrected even in the apochro- 

 matic objectives except by very inconvenient methods of construction ; 

 but whereas the ordinary achromatic objectives are further comphcated 



* ' Bericht liber die wiss. Apparate auf der Londoner International en Ausstellung 

 i. J. 1876 ' (Hofmann) 1878, pp. 415-20. See this Journal, 1884, p. 291. , 



t ' On new methods for improving Spherical Correction, &c.,' see this Journal, 

 1879, pp. 815-7. 



