APLANATIC AND ORTHOSCOPIC EYE-PIECES. 43 



Campani eye-piece. If, for instance, the two lenses, which we have 

 described as the field-lens and the eye-lens, are combined after the 

 method of Eainsden, and if the distance of the principal points 

 turned towards each other equals 20 mm., we obtain for the focal 

 length of the system 24 mm., and for the two principal points 

 E = N + 18 and E* = N + 11, in which N indicates the 

 first refracting surface. The second principal point is situated, 

 therefore, 7 mm. in front of the first. It is, of course, obvious that 

 the optician must bring the eye-point to a convenient position for 

 observation by an appropriate choice of focal lengths. 



The combination of two lenses to form a magnify ing-glass, as in 

 the arrangement of Eamsden, is advantageous, inasmuch as the 

 objective-image is not first diminished and then magnified, but is- 

 magnified by each of them. The final virtual image (encircled by 

 the diaphragm as by a frame) appears, therefore, if the lenses are 

 equal in curvature and aperture to those of the Campani eye- 

 piece, considerably larger than witli the latter ; in other words, the 

 Eamsden eye-piece gives a larger field of view under circumstances 

 in other respects similar, and as it allows of an almost entire 

 elimination of image-distortion, it is especially serviceable for 

 measurements with the eye-piece micrometer. 



These advantages are accompanied by a slight drawback. The 

 fact that the objective-image is very near to the surface of the 

 lower lens causes the slightest defects in the polish, scratches, or 

 particles of dust, &c., on this lens to appear in the field of view, 

 and affect the distinctness of the image. This is not a serious- 

 inconvenience when the eye-piece is carefully cleaned. On the 

 whole we prefer the Eamsden eye-piece to that of Campani. 



c. The Aplanatic and the Ortkoscopic Eye-piece. 



In the catalogues of some opticians we find, besides the ordinary 

 Campani eye-pieces, aplanatic and orthoscopic ones, which are 

 distinguished by a greater flatness of the field of view. Our 

 experience has been limited chiefly to the use of the aplanatic eye- 

 piece of Ploessl, which differs from the ordinary Eamsden eye-piece 

 in being composed of two aplanatic double-lenses ; the total length 

 of the optical system is about 23 mm., and the magnifying power 

 about 4J. The image which is formed by this eye-piece is sharply 



