198 MANAGEMENT OF THE MICEOSCOPB. 



hereafter ; and it will be sufficient in this place to give a table of 

 the average distances of the lineation of different speeies,| which 

 will serve to indicate their respective degrees of difficulty as 

 " tests." The greater part of those which are now in use for this 

 purpose, are .comprehended in the genus Pleurosigma of Prof. W. 

 Smith, which includes those Naviculm whose "frustules" are dis- 

 tinguished by their sigmoid (S-like) curvature (§ 184). 



Lines in 1-lOOOth of an incli. 



1. Pleurosigma littorale, .... 24 



2. Pleurosigma Hippocampus, ... .SO long., 40 trans. 



3. Pleurosigma strigile, .... 36 



4. Pleurosigma strigosum, .... 44 



5. Pleurosigma elongatum, . . 48 



6. Pleurosigma angulatum, .... 52 



7. Pleurosigma Speiiceri, .... 55 long., 50 trans. 



8. Pleurosigma fasciola, ... 64 



9. Pleurosigma obscurum, .... 75 



10. Pleurosigma macrum, .... 85 



11. Nitzschia sigmoidea, ... K5 



12. Navieula rhomboides, .... 85 



The first seven of the foregoing may be resolved, with judi- 

 cious management, by good l-4th or l-5th in. objectives; the 

 remainder require the l-8th or l-12tli in., for the satisfactory ex- 

 hibition of their markings. Several very difficult tests of this 

 desci-iption have been furnished by Prof Bailey of West Point 

 (U. S.), among them the very beautiful GrammatopJiora subtilis- 

 sima and the Hyalodiscus subtiUs; the latter, being of discoid 

 form, and having markings which radiate in all directions, very 

 much like those of an engine-turned watch, is a useful test for 

 observers who have not facilities for obtaining oblique light in 

 any direction ; since, whatever may be the azimuth from which the 

 oblique pencil may proceed, some portion of the disk will always 



' This table is taken from Prof. W. Smith's admirable Monograph on the Diatomaceffi; 

 and it includes most of the species usually employed as tests. These should always be 

 mounted between two pieces of thin glass, according to the method hereafter to be 

 described (§ 122), in order to avoid, as mucli as possible, the production of aberrations 

 in the illuminating pencil. The number of lineations must be considered as an average, 

 the extremes sometimes varying to a considerable amoiuit on either side. A much 

 higher estiinate is given by Messrs. Harrison and Sollitt in the " Quart. Journ. of Mi- 

 crosc. Science," vol. ii, p. 62 ; the Pleurosif^ma fasciola being reclconed by them to contain 

 90 lines in 1-1 000th of an inch, the NKzsihiii sigmcridea 100 lines, and a species cited as 

 Navieula arms (\vhioh can scarcely be the one so named by Ehrenberg, and termed by 

 Prof W. Smith Eunolia arcus) no less than 130. The last they speak of as "so ex- 

 tremely difficult, that, in order even 'to catch a glimpse of its delicate rnarldngs, the 

 observer must be in possession of glasses of a very large angle of aperture and the finest 

 definition, have the most careful management of oblique light, and in addition be pos- 

 sessed of a large share of patience." The Author cannot but believe that there is some 

 error in these measurements; since, as the well-defined lines upon Nobert's test-plate 

 have not yet been resolved, when they have approximated more closely than the highest 

 numbers mentioned in Prof W. Smith's table, it can scarcely be imagined possible that 

 the delicate markings of a Navieula should even be "ghmpsed," if they be as much 

 closer than those of the species previously accounted most difficult, as those of the latter 

 are than those of the easiest. 



