rami 



ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 303 



positives of AmpJiipleura pelliteiJa x 730, the transverse striae on which 

 count 126 to the inch. The wooden case in which the positive is placed 

 carries a Zeiss achromatic lens (No. 127) X 6, focused on the photograph. 



The interference phenomena are as follows : — When the photograph 

 is viewed through the lens, the illumination being of some extent, such 

 as diffused daylight from a white cloud or 

 wall, opalescent globe, &c, the transverse 

 striae appear as in fig. 58 ; but when the source 

 of light is of smaller dimensions, such as a 

 common Microscope lamp with a half-inch 

 wick, the striae are seen as in fig. 59. 



This change of appearance cannot be Fig. 59. 



accounted for by the non-admission of the 

 pairs of diffraction spectra of either the 1st, 

 2nd, or 3rd order, because the angular diver- 

 gence of the 1st diffraction spectrum from the 



dioptric beam is about as many minutes of arc as the lens has degrees 

 of aperture. 



Any moderate difference in the relative size of the stria? and inter- 

 spaces would not alter the case, for Prof. P. G. Tait states that ' the 

 ratio of the breadths of the bar and interstice has but little effect on 

 the result unless it be either very large or very small.' Neither does 

 it matter if the glass side or the film side is next the lens. 



Now we come to a very curious point, viz. that other glass positives, 

 printed from the same negative, do not possess the unique peculiarity 

 which this one has. It is true that, by alteration of focus or other 

 manipulation, the above and other diffracted images may be made ; but 

 I am aware of no object but this one that possesses the peculiarities 

 above described. The negative from which this positive was printed 

 does not exhibit the phenomena in such a striking manner, and then 

 only by an alteration of focus. 



I have another negative x 1200, which will show the effect, but the 

 lens, which has a focus of lj in., requires to be altered either 1 inch 

 within or without its focus before it will show it." 



Spectra of Pleurosigma angulatum.— This subject is a veritable 

 pons asinorum to Mr. E. M. Nelson, who in a further note repeats the 

 mistake on which we commented in this Journal, 1886, pp. 692-5, and 

 which we then described as the most typical instance known to us of a 

 critic being hoist with his own petard. 



To understand Mr. Nelson's new note * it is necessary to recall the 

 original one. 



Mr. Nelson there f expressed his astonishment on two points. The 

 first was that " the E. M. S." should be so foolish as not to see that 

 Dr. Eichhorn's views on this subject " stultified Prof. Abbe's magnificent 

 diffraction theory." 



We pointed out that it was from Prof. Abbe that Dr. Eichhorn's 

 paper was received, and that the problem solved was set by the Professor 

 himself! It was therefore obvious that there was some little mistake 

 somewhere in Mr. Nelson's views. 



The second point was that Mr. Nelson declared (giving what he 

 evidently considered irrefragable reasons for his assertion) that the niark- 



* Engl. Mech., xlvii. (1SSS) p. 32. f Ibid., sliii. (1SS6) p. 337. 



Y 2 



