in common use as Test- Objects. 125 



the extreme difficulty of counting lines so delicate, even grant- 

 ing them to be visible under the lens. Accepting the measure- 

 ments as approximately correct, it is apparent that a series of 

 markings, the variation of which, in the several species, ranges 

 from about 5 to 50 per cent., ought no longer to be received as 

 affording standard tests or specific characters. 



I have stated that the hitherto received interpretation of va- 

 rious structures, as seen under the microscope, has in many 

 cases turned out to be inaccurate, owing to the improved means 

 of observation at our command. I would draw attention more 

 especially to one of the most valuable, inasmuch as it is the most 

 constant in its lineation, of all our test-Diatoms, viz. P. angula- 

 tum. It is well known that, under the generally adopted view, 

 the markings consist of hexagonal depressions on the surface of 

 the valve. Not only has this interpretation been insisted on by 

 all our chief authorities, but its accuracy has been made to ap- 

 pear unimpeachable on the evidence afforded by a photographic 

 image. Facts are stubborn things to contend against ; and in 

 an endeavour to correct an impression based on such apparently 

 incontrovertible testimony I am painfully aware that I have a 

 hazardous task to perform. 



Professor Carpenter (' The Microscope/ p. 304) writes as 

 follows : 



" In the first place it may be remarked, that there is a much 

 greater uniformity in the general character of these markings 

 than was supposed when attention was first directed towards 

 them ; for what were at first supposed to be lines are now re- 

 solved by objectives of large angular aperture into rows of dots ; 

 and these dots, when sufficiently magnified, are found to bear a 

 close resemblance to the coarser markings on the larger species. 

 It is to the latter, therefore, that we should have recourse for 

 the determination of the nature of these markings; and we 

 cannot resort to better illustrations than those that are afforded 

 by Isthmia, Triceratium, and Biddulphia, in all of which the 

 structure of the valve can be distinctly seen under a low magni- 

 fying power, and with ordinary light." After proceeding to 

 show that the markings, in each of these instances, consist of a 

 number of areolar depressions, he continues : " Now, it would 

 not be difficult to bring together a connected series of Diato- 

 macea3 in which the markings, still exhibiting the same general 

 aspect, become more and more minute, requiring for their reso- 

 lution the use of oblique light, or stops with a central diaphragm, 

 and of objectives of larger and larger angular aperture, until 

 we come to those species which present the greatest difficulty, 

 and the nature of whose markings seems to be most obscure. 

 The more perfectly these markings are defined, however, in any 



