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IV, — Measurements of MoUer's Diatomaceen-Probe-Platten. 

 By Edward W. Morley, Hudson, Ohio, U.S.A. 



{Bead before the Eotal Microscopical Society, March 1, 1876.) 



Since Moller's Diatomaceen-Probe-Platte has to some extent become 

 a standard of reference among mieroscopists, an estimate of the 

 variabihty of the fineness of striation on its series of diatoms may 

 have a certain interest for those who own or who have occasion to 

 use the Probe-Platte. 



It is obvious that the fineness of striation on an individual 

 frustule of a given species of diatom mounted in balsam is by no 

 means the only circumstance which influences the ease of its resolu- 

 tion with a given optical power and a given manipulative skill. 

 Even if the same frustule be supposed to be remounted under 

 difi'erent circumstances, differences in the refractive power of the 

 balsam used may occasion slight differences in what may be called 

 absolute resolvability ; and different thicknesses of covering glass 

 and of overlying balsam, by permitting a given objective to work 

 under circumstances more or less favourable to its best performance, 

 may occasion slight differences in relative resolvabihty. Further, 

 if two frustules of the same species and of the same fineness of 

 striation are compared under the same circumstances, their resolva- 

 bility may be greatly afi"ected by differences in the abruptness of 

 the elevations and depressions which appear as strias. The effect 

 of causes like those now mentioned is obviously little capable of 

 numerical statement. 



Another cause which afi'ects the resolvability of a given diatom 

 by a given optical power, is the variation in the fineness of striation 

 of different frustules of the same species. In the case of prepara- 

 tions like the one under consideration, where the refracting power 

 of the balsam used may be assumed to be the same, where the 

 thickness of the covering glass and overlying balsam is found to 

 vary but slightly, and where it may be fairly assumed that the 

 frustules of the same species on different Platten are selected from 

 a somewhat homogeneous stock possessed of common characteristics 

 and differing chiefly in the size obtained before growth was stopped, 

 the difference in the fineness of striation will correspond tolerably 

 with differences in the distinctness of individual striae, and will be 

 a better index of relative resolvability than if diatoms from widely 

 different sources were compared. 



For these reasons, as well as for the reason that the fineness of 

 striation of the diatoms under consideration has a certain intrinsic 

 interest, the writer has measured the strise on the diatoms of ten 

 of Moller's Probe-Platten. The measurements were all made with 



VOL. XV. E 



