16 



Regarding the Apochromat 2 mm, num. Ap. 1.30, with fixed stop for dark-ground 

 illumination for Investigation of Ultra-microscopical Particles, see Prospectuses M 163 

 and M 164. 



As to the kind of immersion fluid to be used with homogeneous 

 immersion objectives see pp. 10 and 11. 



By "Free working distance" as specified in the foregoing table is to be under- 

 stood the distance between the upper surface of a cover-glass 0.17 mm thick and the 

 lower surface of the lens mount, the lens being sharply focussed for an object situated 

 immediately below the cover-glass. As this quantity depends upon the metil mount of 

 the front lens, it is necessarily subject to unavoidable slight variations, and the values 

 given in the table are, therefore, only approximate. 



The diameter of the visible area of the object is governed by the size of the 

 diaphragm of the ocular. The table specifies in millimetres the visible circular area of 

 the object in the case of each combination of component objectives and oculars. These 

 values, too, are slightly fluctuating, as minor variations in the diameter of the diaphragm 

 are also inevitable. 



-- dTi 



w 



Compensating Oculars. 



All objectives of considerable aperture, from their peculiar construction 

 (hemispherical front lens), exhibit certain colour defects in the extra- 

 axial portion of the visual field (chromatic difference cf magni- 

 fication, See DlPPEL, "Das Mikroskop", 2 nd Edition, Brunswick 1882, pp. 225227 : 

 CZAPSKI, "Theorie der optischen Instrumente", 2 ml Edition, Leipzig lldU, pp. 182, 

 368). The differently coloured elementary images which combine to form 

 the final image are of different magnitudes, the blue being larger than the red. 

 Whether an image be projected by such an objective without oculars, or whether 

 it be examined with one of the ordinary oculars, colour fringes will be observed, 

 increasing towards the margin of the field. 



This peculiarity is shared by the apochromatic objectives also, and to their 

 lower powers it has even been intentionally imparted to an approximately similar 

 degree, a means being thereby obtained of almost entirely eliminating this 

 error with the aid of suitable oculars. For this purpose the latter are 

 made to possess an equivalent error of the opposite sign, that is, the 

 image formed be the red rays is larger than that corresponding to tho blue rays. 

 Such oculars serve thus to compensate the errors of the objectives and the 

 images then appear uniformly free from colour up to the very edge of 

 the diaphragm prescribing the limits of the visual field, whilst this edge of the 

 diaphragm itself shows a reddish or yellowish border. 



