15 
patterns of interference are exhibited. The waves of inter- 
ference depend upon— 
(1) The angles of intersection of the superimposed wefts ; 
(2) The fineness of the meshes; and 
(3) If two layers of different coarseness are used, a new 
order of waves are introduced. 
The patterns are endless ; irregular weaving of the gauze 
enhances their beautiful effects. There will be seen endless 
examples of patterns—squares, wavy plaid, Scotch plaid, 
tidal sand ripples, short black lines, &c. &c. 
Examining the latter with a fine three-inch focus crossed 
lens, bright lines of light appear on the black background 
of contiguous wires parallel in the same plane. 
Now, it cannot be doubted that what can be seen in the 
coarse way must take place in the microscopic field. Many 
objects are striated longitudinally and at the same time 
obliquely, forming veritable meshes and delicate waves. 
Again, it is known that light issuing through fine gratings 
suffers diffraction, and, permeating attenuated films, suffers 
physical changes (due, it is supposed, to collision or inter- 
ference waves), generating the most gorgeous colouring, 
sparkling with a peculiar prismatic beauty according to the 
thinness of the structure. ‘These phenomena happen both 
by refraction through films and reflection, and by reflection 
also from a surface broken up with fine lines, as in Barton’s 
buttons, diatoms, and the more resplendent butterflies. 
Without further expansion of this subject (a rich field of 
research), I may be allowed to pass on to a few examples of 
false reading from various causes. 
Error A.—The provisional appearance of a row of spherical 
beads too minute to be defined by our glasses is a cylindri- 
cal figure. For the same reason as the last the shadows 
coalesce. 
The penumbre of spherical error overlap and produce 
the provisional cylinder, as in the conventional form of the 
striated Lepisma saccharina. And when a substratum of 
beading exists, their compound shadows render the delicate 
beading difficult of observation.’ 
Error B.—The intersection of minute cylindrical bodies 
in general produces no spectra or black bands, nor prismatic 
colours, in the best modern objectives. 
If minute threads of spun glass be carefully examined, 
1 Inferior glasses still show the diatom Jizes as ribs or cylindrical appear- 
ances, and the Podura markings as corrugations! grooves, hollows, and 
ridges : often seen when, with immersion lenses and a cracked cover, the water 
is insinuated between the glass and the structure. 
