ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 143 



or will ever see, the complete diffraction-spectra arising from a 

 structure of this minuteness, nor will any Microscope ever show an 

 enlarged copy of it, so long as the spectra cannot be observed in a 

 medium of at least 5 • refractive index, and by an objective of 5 • 

 N.A., which, as far as our present knowledge goes, is an impossibility. 

 The Microscopes of the present day admit relatively a small central 

 portion of the whole diffraction-pencil of the valve — i. e. the incident 

 beam and the six spectra of the inner circle. But this portion is 

 also yielded by a multitude of other objects which are endowed with 

 an alternation of superficial or internal molecular structures which 

 cross each other in two different directions at an angle of 60°. Such 

 structures may be formed in various widely different ways ; it may 

 be by rows of spherules or other prominences of any shape whatever ; 

 rows of internal vacuoles of any figure, or the mere internal alterna- 

 tions of molecular aggregations within a perfectly transparent 

 and smooth silica film. And yet all of these yield with central 

 light the identical circular field of the angulatum valve, even to the 

 most minute particular. But although these spectra are identical as 

 far as the six inner spectral beams are concerned, they may be vastly 

 different in regard to some or all of the more widely diffracted pencils 

 which are not admitted by the objective.' 



However expert, therefore, a microscopist may be (and every one 

 knows the high point which Mr. Nelson has reached), he must not 

 delude himself with the notion that perfection in technical dexterity 

 enables him to determine the " true " structure of objects whose real 

 structure cannot be revealed with our present appliances by any 

 amount of manipulation. The greater his own reputation in this 

 respect, the more undesirable it is that he should proclaim such mis- 

 leading views, to the perplexity of his less experienced brethren." 



Resolution of Amphipleura pellucida by Central Light. — This 

 has been the subject of some controversy in America. Mr. A. Y. Moore* 

 considers the real explanation of the resolution when the mirror is 

 central to be that the edge of the front cell of the objective radiates 

 the light and all light reaching the bottom of the slide at a greater inci- 

 dence than the critical angle is reflected upwards and enters the lens 

 after having passed through the diatom. 



Dr. H. J. Detmers "j" considers this explanation to be quite untenable 

 and the true cause to be that " the resolving rays are reflected from 

 the (externally convex) -internally concave surface of the edge of the 

 immersion fluid." 



Prof. A. Y. Moore, in reply,J insists upon the correctness of his 

 view and the insufficiency of that of Dr. Detmers, inasmuch as the 

 field of view takes the colour of the metal of which the front 

 cell of the objective is made. This would not occur if the light were 

 reflected from the edge of the drop of immersion fluid. 



* The Microscope, iii. (1883) pp. 49-51 (1 fig.). Cf. this Journal, iii. (1883) 

 p, 595. 



t Ibid., pp. 197-201. 

 X Ibid., pp. 201-4. 



