ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 791 



3400 stride to a millimetre, whilst the longitudinal striae are 3600. 

 By the same measure, Navicula crassinervis has 1400 transverse 

 strife, whilst the longitudinal are not less than 2400. "Who can say 

 now that the three types are only one species ? Up to the present 

 time there is no positive reason or experience which can authorize our 

 admitting that a diatom presents such an irregularity in its striation 

 that a frustule has the transverse stri« notably finer than the longi- 

 tudinal, whilst another frustule of the same species has an entirely 

 opposite arrangement. It is true that there exist species whose striae 

 are distributed in a singularly irregular manner on the surface of the 

 valve, Eunotia formica Ehr. for example, but the distribution of the 

 lines in the three types above referred to shows, on the contrary, an 

 admirable regularity over the whole surface of the valves. 



But having condemned as leading to error the use of the eye-piece 

 micrometer for counting the strife, a process to which I have attri- 

 buted for the most part the divergences in the determination by dif- 

 ferent observers of the number of the strife, having then spoken of the 

 number as counted by myself in the transverse and longitudinal 

 directions on three different Naviculce, one of which is the most 

 difficult to resolve of any I know, it may be asked what process I 

 have followed — which is one so exact that I am able to disagree with 

 microscopists of incontestable authority ? This question is very 

 proper, and I hasten to reply to it so that every one may judge if I 

 am wrong in relying on the correctness of my figures, and so that the 

 exactness of my process being once recognized all may profit by it. 

 I use the ordinary process of photo-micrography, and I have made 

 collections of about 3000 photographs of diatoms under the uniform 

 amplification of 53-5 diameters. I employ negatives on glass, and 

 with a suitable apparatus I project the photograph of the diatom on 

 the opposite wall of my laboratory enormously enlarged. I then put 

 over it a piece of paper cut so as to represent exactly -j-L_ riim. under 

 the same conditions, that is to say, taken from the photo-microgi-aphic 

 image of a millimetre divided into 100 parts and enlai'ged to 535 

 diameters. Superposing the paper on the clearest and most regular 

 part of the image, I count or mark each stria or granule, and the 

 number obtained multiplied by 100 gives me the number comprised 

 in a millimetre either longitudinally, transversely or obliquely, 

 according to circumstances. I do not think it can be denied that this 

 method is a very accurate one ; the possible error is hardly worth 

 notice. It is true that the process implies the employment of photo- 

 graphy, which, although it can be of great assistance to the natu- 

 ralist and microscopist, is not yet within reach of every one, nor in 

 habitual use with any. With less convenience, however, one may 

 arrive at the same result by using the camera lucida, or better 

 still, by projecting the diatom directly by means of a good solar 

 microscope. 



I have employed this process of measurement to enable me to 

 form a correct opinion on the number of strife, its constancy or 

 variability in a given species, and to enable me to determine whether 

 it was able to furnish a reliable character in the determination of 



