NOTES AND MEMORANDA. 87 



longitudinal strife of Dippel, however, he regards as diffraction pheno- 

 mena, similar in character to the longitudinal lines which some have 

 described in the central portion of Grammatophora frustules; they 

 varied too much in their distance apart, with varying obliquity of 

 illumination, to bear any other interpretation. The transverse stria?, 

 on the other hand, he found very definite in character. He counted 

 on different frustules from eighty-five to ninety to the thousandth of an 

 inch, which agrees substantially with the results of Dippel, whose 

 figures correspond to from eighty-six to eighty-nine to the thousandth of 

 an inch. The frustules themselves varied in length from -0018 to 

 • 0029 inch. He subsequently removed the cover of one of the dry slides 

 obtained from Moller with the diatoms adherent to it, and mounted the 

 specimen in Canada balsam. The striae were then paler than before, 

 but he is unable to say that he found them more difficult to resolve. 

 Both in balsam and dry he got resolution by the Tolles' immersion ^th 

 belonging to the Museum, and that by lamplight as well as by mono- 

 chromatic sunlight. With immersion objectives of higher powers the 

 lines were still more distinctly separated, and he obtained the finest 

 results with the immersion front of the T yth of Powell and Lealand, 

 and with the new immersion y^th recently made for the Museum by 

 Mr. Tolles. On the whole, he thinks the Frustulia Saxonica an easier 

 test than the Amphipleura pellucida, as may be inferred from the 

 above measurement of its stria?, and the difference is especially marked 

 by lamplight. Those therefore who work by lamplight only will find 

 this test more extensively useful than the Amphipleura. The photo- 

 graph, or rather the Woodbury print which accompanies the paper, is 

 a very good one. It shows the transverse stria? most distinctly ; but, 

 of course, parts of the diatom, which is magnified 1750 diameters, are 

 somewhat indistinct. 



Shall Microscopic Specimens be Photographed, or Drawn by 

 Hand ? — On this question a note appears in the ' American Natu- 

 ralist ' (December, 1872). In the September number of that journal 

 there appears an article entitled " Photo-mechanical Printing," giving, 

 says 0. S. [Mr. Charles Stodder, we believe], " some of Dr. Wood- 

 ward's ideas, and an editorial dissent from them. Now this difference 

 of opinion relates to a point that ought to be settled by the judgment 

 of microscopists, and I write this for the purpose of calling for their 

 views of the question. I quote from the article : — ' Even the micro- 

 scopist himself, being unable to represent all that he sees, is obliged 

 to select what he conceives to be of importance, and thus represents 

 his own theories rather than severe facts ' (Dr. Woodward). The com- 

 ment is [' If, however, his theories are correct, and his delineation 

 skilful, this very power of selection and construction enables him to 

 give a distinctness and completeness which is lacked by the photo- 

 graphic camera.'] Here are two almost opposite principles of illustra- 

 tion in question. Which should be the governing one ? What is the 

 object of the pictures ? Obviously there are two — one for explanation 

 of the observer's theories ; the other, that other observers may, in re- 

 peating the observation, be guided by and recognize what the first one 

 had seen, and this I consider the all-important object of ' figures.' If 



