MICRO-OBJECTIVES AND COMPENSATING OCULARS. 157 



objectives of relatively long foci, thus obviating 

 the necessity for objectives of extreme short focal 

 lengths. 



(3) By the correction of the secondary spectrum 

 and the perfect spherical correction of these new 

 lenses, the visual and actinic foci coincide, rendering 

 them especially suitable for photo-micrography. 



(4) The increased spherical and achromatic correc- 

 tions of these objectives produce a larger concen- 

 tration of light in the images projected by them. 



Dr. Abbe calls these new lenses Apochvomatics 

 or Apochromatic Objectives, owing to their superior 

 spherical and achromatic corrections, which repre- 

 sent an achromatism of a higher order than hitherto 

 attained. 



These apochromative objectives require special 

 eyepieces in order to utilise their capabilities to 

 the fullest extent, and Dr. Zeiss has constructed 

 suitable eyepieces for them which he designates 

 Compensating Oculars. 



In objectives of short focal length or in high 

 powers the front lens is generally a single crown 

 glass lens, which is, therefore, unachromatic, the 

 result being coloured outlines of the image in the 

 marginal zone, as the lens is only well corrected 

 for its central portion. The front lenses of the 

 high -power apochromatic objectives are evidently 

 also single ones, and the so-called compensation 

 oculars have been constructed with a view to correct 

 this residue of peripheral aberration, and to balance 

 or compensate the chromatic differences of magni- 

 fication, as the picture produced by the blue and 

 violet rays is larger than that produced by red and 

 yellow rays. In order, therefore, to make the com- 

 pensation oculars available not only for the high 

 objectives but also for the lower ones, the latter 

 had to be so constructed that the difference of the 

 chromatic magnifications of the marginal zone should 

 be practically the same as in the former. These 

 compensation oculars differ from the ordinary Huy- 



