ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 789 



striation did not require the highest powers of the Microscope and the 

 most delicate disp(;sition of the illumination, there was not much 

 difficulty in determining the number of the stride contained in a given 

 space ; but when the progressive improvement of the Microscope 

 enabled the granulations of the Grainmatojphora, Pleurosigma, Nitzschia, 

 and ArnijJupleura to be distinguished, the attempts which different 

 observers made to measure the fineness of the sculpturing led to very 

 discurdant results. The authority of these observers prevented the 

 differences in the measurements from being attributed to errors, and 

 this was the origin of the opinion that the number of the strife com- 

 prised in a given space is not constant, and could not constitute a 

 proper character for the determination of species. But the divergence 

 in the results obtained often arose from the fact that types had been 

 taken for measurement which were not really identical, and did not 

 belong to the same species or variety. Moreover, one method of 

 measurement had deplorable consequences. It is thus, for example, 

 that Messrs. SoUitt and Harrison, of Hull, in affirming that they 

 had found that the number of the transverse striae of ArnjjJiijpleura 

 pellucida is not less than 120,000 per English inch (52-.0 in a 

 millimetre), have certainly fallen into error, for notwithstanding 

 the important progress which has been made in late years in the mag- 

 nifying power of the Microscope, we have not arrived at anything 

 beyond the definition of the strise, and moreover, such a number of 

 striai notably surpasses the limit of visibility, as Professor Helmholtz 

 has recently established. The method of measuring by an eye-piece 

 micrometer would be the best (as certainly it is the most rapid), if at 

 the same time it was free from error. But when we deal with details 

 of an excessive fineness, and such as almost defy the resolving power of 

 the most perfect and powerful objectives, it is extremely diflicult, and 

 leaves room for a great deal of error, to determine with precision the 

 exact number of stride comprised in a division of the eye-piece micro- 

 meter, the number of which is the smaller as the magnifying power is 

 the greater. When we multij^ly this number of strife comprised in a 

 division of the micrometer by the value of that division, the probable 

 error will become so much the greater, and such that the result 

 may be far removed from the truth. It is thus that the two 

 microscopists above named have been led into error by the employ- 

 ment of a bad system of measurement as regards the strife of the 

 AmpJiipleura, the number of which they have given much beyond the 

 truth. 



Thus the difficulty in the determination of the number of strias 

 in a given space, on the valves of the Diatomaceee, the disparity in 

 the measurements made by different observers, and, at the same 

 time, the fancy of those who will not admit the existence of species 

 (these, although devoid of every positive argument, and not basing 

 their objections upon experience, aftect to consider every organic form 

 as accidental, and as a transitory state of an organism in actual and 

 incessant evolution), all these circumstances have contributed to their 

 denial of the value as regards specific character in the number of the 

 stricB on the valve of a diatom. 



