ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 141 



areolation of the Coscinodisci, which can only be seen by direct light ; 

 for with oblique light the blur of the hexagonal structure blots out 

 the fine markings. When we come to the very finely-marked diatoms, 

 such as A . pellucida and some of the Nitzschice, we must be content 

 with lines, by oblique light, until we can get sufficient aperture to 

 enable us to see the ultimate structure." 



" Monachus " rejoined as follows : * — " I am obliged to Mr. Nelson 

 for his reply to my letter, as it leaves no room for ambiguity as to 

 his views. 



It is not of course my object, in occupying your space, to simply 

 engage in a personal controversy with Mr. Nelson, and I therefore 

 leave, for the moment at any rate, many points in his letters in regard 

 to which he is mistaken, such as the statements about the two beams 

 in the case of N. Bhomhoides, the lines and dots he figures, &c. My 

 object is to prevent your readers being misled on the cardinal 

 statement of Mr. Nelson that he (or any one else) has seen the true 

 structure of Surirella gemma, or any similar diatom. When this is 

 seen we shall have reached the millennium of microscopical observa- 

 tion — how far we are from that day no one can tell, but it is certain 

 we have not reached it yet ; and in representing what he saw as the 

 ' true structure,' Mr. Nelson was but falling into the same error as 

 the old school of microscopists whom he criticizes. 



I will first quote Mr. Nelson's statement verbatim : — ' If S. gemma 

 is examined by this means, the hemispherule theory is at once exploded, 

 and the true structure (which is far more beautiful) is revealed. It is 

 something like a most delicate skeleton leaf.' 



Why S. gemma is beyond the reach of any such determination of 

 its true structure, it is the object of the succeeding paragraphs to 

 show. 



When rays emanating from a luminous body are transmitted 

 through any structure, which by its opaque, semi-transparent, or 

 refractive constituents prevents the continuous propagation of the 

 luminous waves, the rays cease to pass through in straight lines, and 

 each pencil is split up into a conical pencil of rays, which are dis- 

 tributed round the course of the incident pencil, and which vary very 

 much in the extent of their deviation. 



When the elements of the structure are considerable multiples of 

 a wave-length, that is, when they are relatively large, the spread of 

 the diffracted rays is limited ; but when the elements are only very 

 small multiples of a wave-length, that is, when they are very minute, 

 the diffracted rays are spread out very widely. 



Most microscopists are by this time familiar with the practical 

 effect of the diffraction-spectra imder the Microscope, and have seen 

 the experiments which show that the same diatom will give numerous 

 very different images according as we admit all or some only of the 

 diffraction-spectra. By stopping off successively the seven spectra, 

 for instance, of P. angulatum, we get as many different structural 

 appearances — indeed, no less than nine different sets of lines may be 



* Engl. Mech., xxxviii. (1883) p. 431 (1 fig.). 



