10 



I'.y "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 focused for an object situated 

 immediately below the cover-glass. As this quantity depends upon the metal 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 eye-piece. The table specifies in millimetres the visible circular area <>f 

 the object in the case of each combination of component objectives and eye-pieces. These 

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

 are also inevitable. 



* 



Compensating Eye-pieces. 



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 of magnification, see 

 DIPPEL, "Das Mikroskop", 2^ ed., Brunswick, 1882, p. 225227; CZAPSKI, 

 "Theorie der optischen Instrumente" , Breslau, 1893, p. 134). '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 eye-piece, or whether it be examined 

 with one of the ordinary eye-pieces, 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 eye-pieces. For this purpose the latter are 

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

 image formed by the red rays is larger than that corresponding to the blue 

 rays.\Such eye-pieces 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. 



