ON THE THEOLir OP TSE MICROSCOPE. 241 



delineation of structure imaged by any particular object under 

 any definite illumination, if only the actual efi'ective diffraction 

 phenomena be given, namely, the number, disposition, and 

 relative brightness of all the diffraction spectra. 



XVIII. The final result of these researches may be thus 

 stated. — 



Every thing visible in the microscope picture which is not 

 accounted for by the simple '' absorption image," but for which 

 the co-operation of groups of diffracted rays is needed — in fact 

 all minute structural detail — is, as a rule, not imaged geometrically, 

 that is, conformably with the actual constituent detail of the object 

 itself. However constant, strongly marked, and so to speak 

 materially visible, such indications of structure may appear 

 {e.g. J striae, or mapped out fields, &c.) they cannot be interpreted 

 as morphological, but only as physical characters ; nat as images 

 of material forms, but as signs of certain material differences of 

 composition of the particles composing the object. And nothing 

 more can he safely inferred from the microscope revelation than the 

 presence, in the object, of such structural peculiarities as are necessa/ry 

 and adequate to the production of the diffraction phenomena on which 

 the images of minute details depend. 



The smaller the linear dimensions of structural elements are, 

 the fewer in number will be the diffraction pencils which come 

 into operation even with the largest possible angle of aperture : 

 the less effectively can the gradation of intensity in the series of 

 these diffraction rays bring into view such structural differences 

 as are still possible within the same relation of dimensions ; and 

 the more indefinite will be the conclusion to be drawn from the 

 image, or even from any visible diffraction phenomena, respecting 

 the true structure of the object. 



Erom this point of view it must be evident that the attempt to 

 determine the structure of the finer kinds of diatom valves by 

 morphological interpretation of their microscopic appearances, is 

 based on inadmissible premises. Whether for example, 

 Pleurosigma angm. possesses two or three sets of striae j whether 



