294 ANGULAR AND NUMERICAL APERTURE CCh. IX 



constructed to give a red image sufficiently large to bring its mag- 

 nification up to that of the blue image, and hence the final image as 

 seen by the eye is correct. The low power apochromats could be 

 corrected for this, but for the sake of using the same oculars on aU 

 powers the defect is left or purposely introduced into all the apochro- 

 mats. It will be seen from the above statement that for projection 

 or for photography the apochromats cannot be used satisfactorily 

 without the ocular to complete the corrections (see fig. 174-175). 



The over-correction of the ocular necessary to give the greater mag- 

 nification to the red constituent of the image leads to the position of 

 the red on the outside of the projected (virtual) beam; hence in looking 

 through a compensation ocular toward the window or the sky an 

 orange haze appears around the margin. As the ordinary Huygenian 

 ocular has an under-corrected eye-lens the blue constituent will be on 

 the outside of the projected (virtual) image and there appears a 

 blue haze around the edge of the field (Spitta, p. 11 2-1 13). 



Angular and Numerical Aperture 



§ 467. Angular aperture. — By this is meant the angle of light 

 which passes from the object to the objective and becomes effective 

 in producing the microscopic image (fig. 176). It has been known for 

 a very long time that the clearness of the image, other things being 

 equal, depends upon the width of the angle of light coming from the 

 object; and that the resolution of details depended very largely upon 

 the angular aperture of the objective. The difficulty of overcoming 

 the aberrations also became greater as the angle was increased; and 

 it was the triumph of the early American opticians, Spencer and 

 ToUes, that they were able to make the corrections for high powers 

 with very large angular aperture. 



§ 468. Numerical aperture. — With the introduction of immersion 

 systems into modern microscopy, it was seen and pointed out with 

 great distinctness by Spencer and Tolles that the aperture of such 

 immersion objectives might exceed 180° of Ught in air. For the 

 average microscopist, however, this seemed an impossibility. By 

 referring to fig. 158 to 160 the matter becomes very easily intelligible, 



