18^9] MICROSCOPICAL JOURNAL. 307 



tice is necessary, as much depends on the actual illumin- 

 ation of the moment ; but rapid change of the lenses 

 nearly always affords a ready method of perceiving the 

 superiority of the apochromatic. 



4. Resolving Power. — This is directly proportional 

 to the N. A. of the objective. By resolving power is 

 meant, speaking popularly, the power possessed by the 

 lens of breaking coarse details into still finer ones, and 

 of rendering the fine details so distinct as to merit the 

 term of being -'picked out." The black markings of the 

 tracheae should with the inch look picked out and so sharp 

 and clean-cut, that they ought to give the impression that 

 they are drawn with a pen on paper. Remove now the 

 proboscis and place on the stage a large diatom, such as 

 an A.rachnoidiscus, and notice how the fine markings are 

 shown by each lens, and how white and colorless is the 

 image in the apochromatic. 



5. Absence of Color Fringes.— Around and about 

 the markings of the diatom, and between the strise or be- 

 tween the dots, there will be seen a certain amount of 

 color fringes with the achromatic, which are entirely ab- 

 sent in the apochromatic owing to the elimination of the 

 secondary spectrum. Return now the proboscis slide 

 and look at the large hairs and note the difference in their 

 fringe-like appendages. 



The same specimens will do when comparing the im- 

 ages produced by two half-inches, each being of similar 

 aperture, when it will be found the contrast in the images 

 will be more marked. Use also a diatom with well-mark- 

 ed strise such as a Navicula major. When comparing two 

 l-6ths, a podura scale may be also employed. The light 

 wants careful arrangement to use this test. The mark- 

 ings should look "punched out," and very black with a 

 central white streak ; diatoms should be tried for color 

 fringes, especially the Navicula. An eighth and a twelfth 

 immersion require much more careful testing. A podura 



