168 THE MICROSCOPICAL NEWS. 
The delineation of structure seen in the field of the microscope ts 
in all tts characters,—those which are conformable with the real con- 
stitution of the object as well as those which are not so—nothing 
more than the result of this process of interference occurring where 
all the imageforming rays encounter each other. ‘The relation 
existing between the linear distances from the axis of the micro- 
scope of constituent elements of the aperture image, and the 
various inclination of rays entering the objective, taken together 
with the dioptric analysis of the microscope, afford all the data 
necessary for complete demonstration of the above positions. From 
them may be deduced that in an achromatic objective the inter- 
ference images, for all colours, coincide, and yield as a total effect 
achromatism, thus differing from all other known interference 
phenomena. 
The final result of these researches may be thus stated : 
Everything 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, comformably 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, they 
cannot be interpreted as morphological, but only as physical 
characters ; not as zmages of material forms, but as szgvs of certain 
material differences of composition of the particles composing the 
object. And nothing more can be safely inferred from the microscope 
revelation than the presence, in the object, of such structural 
peculiarities as are necessary and adequate to the production of the 
diffraction phenomena on which the images of minute details depend. 
From this point of view it must be evident that the attempt to 
determine the structure of the finer kinds of diatom valves by mor- 
phological interpretation of their microscopic appearances, is based 
on inadmissible premises. Whether, for example, Pleurosigma 
angulatum possesses two or three sets of striae ; whether striation 
exists at all; whether the visible delineation is caused by isolated 
prominences, or depressions, &c., no microscope however perfect, 
no amplification however magnified, can inform us. All that can 
be maintained is the mere presence of conditions optically necessary 
for the diffraction effect which accompanies the image-forming 
process. So far, however, as this effect is visible in any microscope 
(six symmetrically disposed spectra inclined at about 65° to the 
direction of the undiffracted rays, ordinary direct illumination being 
employed), it may proceed from any structure which contains in its 
substance, or on its surface, optically homogeneous elements 
arranged with some approach to a system of equilateral triangles of 
0'48 dimensions (= circa zz}, inch). Whatever such elements 
