94 APOCHROMATIC OBJECTIVES 



production of the natural colors 



of objects, even of delicate tints, 



as well as the ability to use very 



high power eye-pieces^ without 



breaking down the image, are 



advantages which will be keenly 



appreciated by the scientific 



worker. 



All Zeiss's apochromatic objectives are made with relatively 



wide angles of aperture. A few years ago the contentions among 



microscopists as to the relative merits of high- and low- angled 



objectives were very strong, and even fierce ; but later experience 



from the extended use of the former has much modified the views 



of those whose contentions were in favour of the latter. The 



present opinion of the best experts seems to be that for critical 



ocula?' work the wide-angled glasses are greatly superior to the low, 



while for photographic and some other purposes the low-angled 



glasses with their longer focus have an advantage. 



Prof. Abbe has made a very necessary and useful reform in the 

 designation of the aperture of lenses. The aperture is the mea- 

 sure of the defining power of an objective. The wider the angle 

 the greater the number of image-forming rays passing into the 

 objective ; and conversely, the smaller the angle the fewer the rays. 

 The old term, ' angular aperture ' — which means the angle con- 

 tained between two lines drawn from the focal point to the oppo- 

 site sides of the pencil of light emerging from the back lens of any 

 objective was sufficient to indicate the relative defining power of 

 objectives, so long as air was the medium through which the object 

 was viewed. But when ' immersion ' lenses were introduced, 

 where water or oil became the media in place of air, the differing 

 refractive powers of these fluids rendered the term inadequate. 

 Obviously, the refractive index of the medium intervening between 

 the cover-glass of the object and the front lens of the objective, 

 must be a factor in the determination of aperture. 



These considerations led Prof. Abbe to adopt the term ' Nu- 

 merical Aperture' (N.A.), and to establish for its determination 

 the formula, ?t sin. z^=N.A., where n represents the refractive 

 index of the medium between lens and cover-glass, and sin. u 



