ZOOLOGY AND BOTANY, MI0B0800PY, ETC, 323 



his better informed antagonist. This device has boon applied by 

 Mr. E. M. Nelson to the question of central and oblique light. 



The author first describes Mr. Stephenson's paper (ante, p. 37) 

 as having for its object to discountenance the use of central illumina- 

 tion, and as drawing the conclusion that the central portion of the 

 illuminating beam is " useless, harmful, and as such ought to be 

 stopped out " ! It is hardly necessary to tell any one who has read 

 that paper that Mr. Nelson's description is as little correct as it 

 would be to describe it as a paper having for its object the extraction 

 of central illumination from cucumbers. The demonstration of the 

 absurdity of the supposed view is, of course, under such circumstances, 

 unusually complete. 



Mr. Nelson next combats the view "that nothing can be known 

 about the structure of the Diatomacese, because all the diffraction 

 spectra are not admitted," which is proposed to be refuted by show- 

 ing " that something can be known of the structure of P. forniosum, 

 because some of the diffraction spectra are admitted." In course of 

 time we have come to know a little of the views of theoretical 

 microscopists, but we have not yet met or heard of any one who holds 

 or ever held the viev7 quoted by Mr. Nelson, which we fear has only 

 a subjective existence. We are at a loss to understand why such a 

 misstatement should have been made in what is apparently intended 

 for a scientific paper, and purporting to be written au serieux. It 

 seems to us, with all deference, to serve no useful purpose from any 

 point of view. 



The next point dealt with by the author is put in such a way 

 that to be properly appreciated it must be quoted in extenso. 



" It is curious to note how those who refuse to know anything 

 about the structure of the Diatomaceae, because all the diffraction 

 spectra are not taken up, affirm that a German student, who had 

 never seen a diatom, worked out the purely mathematical result of 

 the interference of the six spectra of P. angulatum. The purely 

 mathematical result is a very simple business. The diffraction 

 spectra are chromatic images of the source of light arranged in a 

 pattern similar to the pattern which causes the interference, the 

 mathematical connection between the spectra and the pattern being 

 in the size of the interspace and the angular divergence of the spectra 

 from the dioptric beam. All that the German student could do was 

 to say that the source of light was a disc, that the pattern causing 

 the interference was similar to the pattern of the diffraction spectra — 

 namely, quincunx — and that if the angular divergence of the spectra 

 from the dioptric beam were given, the size of the interspaces could 

 be found out. Instead of which, he drew a fantastic picture for 

 which there was no warrant from the data given. As this picture 

 had been drawn from purely mathematical investigation, of course 

 the markings must be there, although no one had ever seen them. 

 The angulatum was re-examined, but with a stop at the back of the 

 objective, and the small secondary markings predicted by the 

 German student were seen. The whole affair was given out as a 

 microscopic edition of the discovery of the planet Neptune." 



Y 2 



