54 Transactions of the Royal Microscopical Society. 
light. This diatom mounted dry with the friistules adhering to the 
cover is seen in quite a new aspect ; by causing the pencil of light 
to fall at right angles to the axis of the valve (or more generally 
parallel to any of the three ordinary lines). It then presents at the 
portion adhering to the thin glass cover white rectangular fields, 
separated by dark broad beams, alternating from row to row, paler 
lines crossing the white space between the black bands which are 
thus joined ; the ratio of the length of the rectangular division 
comprised between the dark bands being to the width as 2 : \/3.” 
This unusual image results from the entrance into the field of 
a new set of diffraction images, or rather, of one (say li) of the 
twelve more distant diffraction images surrounding the six spectral 
images (b, c, d, e,f, y), which are always visible on Angulatum 
(with an objective of moderate aperture and central light), on 
removing the eye-piece and looking down the tube,"^ the dotted 
circle in the figure indicating the appearance of the field with 
central, as the darker ring does that of oblique light by which the 
more distant spectrum is admitted. 
O O 
O OLp-''“P0,lO O 
o o o o 
O O 'W-O'" o o 
o o o o o 
With light of the utmost obliquity on this diatom, a moderate 
angle admits only two (say h, e), or at most three, of the spectra of 
the first order, the more distant spectrum (say h), or at least its 
light -giving rays, being inaccessible. 
This is the case also with the oil immersion lens on that part 
of the valve on which a film of air intervenes between it and the 
glass cover : under such circumstances the aperture is reduced to the 
maximum air- angle of 180° = 1 numerical aperture ; hence, on the 
parts not in contact with the cover the ordinary hexagonal markings 
alone are usually seen, but with great brilliancy and distinctness, 
with either central or oblique light. 
Further, Professor Abbe states that on Frustulia Saosonica the 
fine parallel longitudinal lines described by Mr. Hickief are seen by 
* ‘ Monthly Micro. Journal,’ vol. xvii. p. 82. 
t Ibid., vol. xiv. pp. 32 and 274 ; vol. xv. p. 122. 
