The Microscope. 381 



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TECHNOLOGY 



TESTS FOR MODERN OBJECTIVES. 

 1\ /TR. E. M. nelson considers that the advance of the 

 -^~-*- microscope in recent years is due to the Podura 

 scale and the following diatoms: 1st, Rhomboides; 2nd, Gh^am- 

 matophora subtilissima ; 3d, and probably to a greater extent, 

 Anlphipleiira pellucida ; lastly, and at the present time, to Pleuro- 

 sigma angulatum, N. rhomboides, and the secondary markings of 

 diatoms in general with large-angled cones of central light. It was 

 the demand for glasses which would give classical images of the 

 Podura scale which improved the central portions of the objectives, 

 and it was the demand for diatom-resolving lenses which spurred on 

 the opticians to make wide angles and to correct the margins. But 

 however much we may regret it, these old tests, the Podura and the 

 Amphipleura pellucida, which have been of great service to the 

 cause of microscopy, must be laid aside. The classical picture of 

 Podura demands such a very small area of the center of an 

 objective that it tests too little of the glass. The following are a 

 few tests for modern objectives: 



1. Pleurosigma angulatum, showing dark perforations on a 

 light ground, with a fracture passing through them. While the 

 dioptric beam passes through the center of the lens, the diffraction 

 spectra sweep the margin. Unless a lens be truly centered, it will 

 not stand this test. 



2. A Cherryfield Rhomboides in balsam or styrax, with the 

 full aperture of Powell's latest condenser, is a very severe test. 



3. To these may be added the secondary markings on diatoms, 

 e. g., Coscinodiscus aster omphalus, etc. 



4. The fracture passing through the secondary markings, such 

 as (a), Triceratium; (6), Isthuria nervosa. 



5. The secondary markings of the ariolations on the hoop of 

 Isthuria nervosa in balsam. 



All these tests are intended for solid cones of direct light of 

 various apertures. Two classes of tests are comprised in this list. 

 The first, and perhaps the best, is the way a fairly large test is 

 presented; 1, 2, 4 (a), and some of 3, are in this class. The other 

 class consists in the possibility in making out the test at all; 4 (6), 

 5, and some of 3, are in this class. — Journal o^ the Royal Micro- 

 scopical Society. 



