250 Extracts from Mr. H. E. Fripp's Translation of 
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, they 
cannot be interpreted as morphological, but only as physical 
characters ; not as images of material forms, but as signs of certain 
material differences of composition of the particles composing the 
object. And nothing more can be safely inferred from the micro- 
scope 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 
morphological interpretation of their microscopic appearances, is 
based on inadmissible premises. Whether, for example, Pleurosigma 
angulatum possesses two or three sets of striae ; whether striation 
exist at all ; whether the visible delineation is caused by isolated 
prominences, or depressions, Ac., 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/r 
dimensions ( = circa ^hro inch). Whatever such elements may 
be — organized particles or mere differences of molecular aggregation 
(centres of condensed matter) — they will always present a delineation 
of the familiar form. All ground for assuming these elements to 
be depressions or prominences fails, after proof that neither the 
visibility of the markings nor their greater distinctions under oblique 
illumination has anything to do with shadow effects. The dis- 
tribution of light and shade on the surface of the valve in the form 
of a system of hexagonal fields, is the mathematically necessary 
result of the interference of the seven isolated pencils of light which 
is caused by diffraction, whatever may be the physical condition of 
the object causing this diffraction : the position of the hexagonal 
fields, with two sides parallel to the middle ribs, has its sufficient 
reason in the visible disposition of the diffracted spectra towards 
the axis of this valve, and can be deduced by calculation without 
any necessity for knowing the actual structure of the object. 
That the same state of things obtains in numerous instances of 
organic forms, the study of which belongs to the province of 
histology, we may learn from the instance of striated muscular 
