ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
659 
hypothesis which will make it take an intelligible place amongst natural 
objects. 
Our skill as microscopists, apart from the technical dexterity in tho 
use of our tools, consists largely in devising varied experiments and 
changes of condition, so as to enlarge the body of evidence from which 
we draw our inductive conclusions. To assist ourselves in this, we also 
catalogue such facts and methods, and such cautions and warnings, as 
our experience (or that of others) has taught us. Let us look for a 
moment at some examples. 
We know very well that we are liable to illusions of sight, so natural 
and so powerful that even the intellectual certainty that they are 
illusions will not destroy them. If we are looking through the Abbe 
binocular eye-piece, using the caps with semicircular openings, we see 
a hemispherical object as if it were a hollow bowl, and, visually, it 
refuses to be anything else. But this is not peculiar to microscopical 
vision, for we do an analogous thing with the stereoscope, and by 
wrongly placing the pictures may make an equally startling pseudo- 
perspective. 
We find that what we call transparent bodies are full of lines as dark 
as if made with opaque paint, and throw far-reaching shadows. But I 
see similar ones in the cubical glass paper-weight on the table before 
me, and know that by the laws of refraction the surface of a transparent 
body is always dark when its angle to the eye is such as to cause total 
reflection of the light in the opposite direction. By the same law we 
know that if the angle of total reflection in the same transparent cube 
were differently placed with regard to the eye, the now dark surface 
would become a mirror, reflecting the sky and distant objects as 
brilliantly as if silvered. Our diatom-shells give us constant experience 
in these phenomena. A prismatically fractured edge will scintillate so 
as to defy all efforts to define its outline. Reflected images look like 
actual details of structure in the object. Dealing, as we constantly are, 
with objects made of glass, we have constant use for our reasoning 
faculties to determine the meaning of all these refractions and reflec- 
tions, which sometimes are almost as confusing as the broken images 
seen through the glass pendants of a chandelier. 
In addition to these familiar effects of refraction and reflection, we 
have the class of phenomena which we call diffraction effects. These 
may be wave-like fringes of light and shadow following the outline of 
the transparent object, and reduplicating this outline ; or they may be 
analogous fringes thrown off the subdivided parts of the object, as from 
the cup-like outline of alveoli, or from some projecting rib or groove 
like those along the diatom’s median line. 
We know by constant experience that when we throw light obliquely 
through a transparent reticulated object like a diatom-shell, the dif- 
fraction fringes from the separate alveoli run together across the shell 
in dark striae, oblique or at right angles to the direction of the light. 
In the Pleurosigma, in which the rows of alveoli are oblique to the 
midrib, we very easily get the oblique striation by the use of oblique 
light ; getting both series of lines at once, one only, or one strong and 
the other faint, as we please, and with very little trouble. We get, with 
a little more pains, a transverse striation, at right angles to the midrib, 
