DIATOM-TESTS. 



181 



Navicular whose 'frustules' are distinguished by their sigmoid 

 (S-like) curvature (Fig. 120). 



Direction Stricein l-1000th of an inch. 



1. 



2. 



3. 



4. 



5. 



6. 



7. 



8. 



9. 

 10. 

 11. 

 12. 



Pleurosigma f ormosum 

 strigile 



of St rice. 



Smith. 



• Balticum 



• attenuatum 

 hippocampus 

 strigosum 

 quaciratum 

 elongatum 

 lacustre 

 angulatum 

 sestuarii 

 fasciola 



diagonal 34 



transverse 36 



transverse 38 



transverse 40 



transverse 40 



diagonal 44 



diagonal 45 



diagonal 48 



transverse 48 



diagonal 52 



diagonal 54 



transverse 64 



transverse 85 



transverse 85 



transverse 



Sollitt. 

 32 — 20 

 30 



40 — 20 

 46 — 35 

 45 — 40 

 80 — 40 

 60 — 35 



51 — 46 



90 — 50 

 111 — 60 



130 —120 



13. Navicula rhomboides 



14. Nitzschia sigmoidea 



15. Amphipleura pellucida 



(Navicula acus) 



The first column of measurements in the above table gives the 

 number stated by Prof. W. Smith as averages; the second column 

 gives the numbers more recently assigned as the extremes by 

 Mr. Sollitt,* who has pointed out that great differences exist in 



* 'On the Measurement of the Strise of Diatoms,' in "Quart. Journ. 

 of Microsc. Science," Vol. viii. (1860), p. 48. Mr. Sollitt remarks of 

 P. fasciola, P. strigosum, Nitzschia sigmoidea, and Navicula rhomboides, 

 that individual specimens often have the strise so fine as to defy all 

 means of resolving them. On the other hand, it is asserted by Mr. 

 Hendry ("Quart. Journ. of Micr. Science," Vol. i. N.S. (1861), p. 231, that 

 the stria? of N. rhomboides range between 30 and 50 in l-1000th of an 

 inch. It is in regard to Amphipleura pellucida, however, that the 

 greatest difference of opinion exists. By Mr. Hendry it is affirmed 

 ("Quart. Journ. of Micros. Science," Vol. viii. 1860, p. 208; and Vol. i. 

 U.S. 1861, p. 87), that tbe number of its strise ranges as low as 34, and 

 that many specimens present 60, 70, and 80 in l-1000th of an inch ; so 

 that in some individuals the striation may be resolved with a l-5th, a 

 l-4th, a 4-10ths, or even a Half-inch Objective, whilst in others it requires 

 the l-8th, or even higher powers. On the other hand, Messrs. Sullivant 

 and Wormley ("Silhman's American Journal," Jan., 1861, and "Quart. 

 Journ. of Microsc. Science," Vol. i. N.S. 1861, p. 112), question the 

 reality of any real striation in this species, and altogether dispute the 

 possibility of discerning strise whose distance is no more than l-130,000th 

 of an inch ; pointing out with reference both to the Diatom-tests and 

 Nobert's Test-plate, that when the resolving power of an Objective is 

 near its limit, ' spectral ' or ' spurious ' lines are to be seen, only to be 

 distinguished from the true by a practised eye. " The testimony of 

 our Objectives, as we understand it, seems to indicate that this Diatom 

 has a minutely and irregularly broken-up surface, which even on the 

 same valve can be made to show an apparent striation, varying from 

 moderately coarse to extremely fine, according to the obliquity or 

 intensity of the illumination, and to the grade, whether low or high, 

 of the Objective used, thus proving beyond all question that the exhi- 

 bition is illusoiy. In numerous trials, particularly on fine English 

 specimens from Hull, we have entirely failed, with glasses, too, of 

 unsurpassed excellence, to bring out regv.lo.r, distinct, and unmistak- 

 able strise, such as would be at once so recognized by an eye practised 



