ORGANIC TEST-OBJECTS. 137 



lines ought to be resolved into small apparently hexagonal spaces 

 (Fig. 86). This appearance is stated by Floegel 1 to be incorrect, on 

 account of the interference phenomena. 



10. Nitzschia sigmvidea. This long and narrow valve shows 

 fine close transverse stride, which, according to Schacht, were first 

 resolved by Hartnack's No. 10 immersion with oblique light 

 they are, however, visible with the same illumination with the 

 corresponding objectives of Beneche and Zeiss. These conclusions 

 of Schacht refer to dry specimens, which, it is well known, are 

 less difficult than those mounted in balsam. 



11. Grammato2^iora sultilissima (Fig. 87, 1). Marginal portion 

 a with extremely fine and close transverse / 



strire (Fig. 87, 2), which are resolved (if 



the preparation is in balsam) only by 



very good objectives. With Hartnack's 



No. 9 immersion we can scarcely see any 



indication of this striation. The most 



powerful immersion objectives of recent 



construction resolve these stride distinctly, 



and also exhibit isolated traces of oblique 



series of stride, as in Plcurosigma anyulatum and in the larger 



Grammatopkorce. 



12. Navicnla rliomboidcs. With longitudinal and transverse 

 striae, the former more distinct and not very difficult, the latter 

 exceedingly sharp and fine. Was used as N. affinis, as a test- 

 object (balsam mounted) at the London International Exhibition 

 (1862). 



In addition to the preceding test-objects several others have 

 been recently proposed which may be of service ; for instance, 

 different species of Navicula. Coscinodiscm, Frustidia sccxonica, 

 Hyalodiseus subtilis, and others, on several of which a striation as 

 fine as 4 to 5 lines in a micromillimetre should be seen. Our list 

 is, however, sufficiently complete, and we are not inclined to 

 attribute importance to the discovery of new test-objects with our 

 present knowledge of practical Optics. We would advise the micro- 

 scopist to select three or four, and to note most carefully their 

 appearance under different Microscopes. This will always be of 

 more advantage to him in testing an unknown instrument, than 

 the simple fact of whether it resolves a particular test-object or not. 



1 " Botanische Zeitung," 1869, p. 757. 



