14 
§ IV. On Provisional Definition. 
There is abundant reason for supposing that there may 
be, and probably are, a good many, microscopic appearances, 
even those regarded as tests, which must be considered as 
merely provisional until better modes of observation are 
established. 
Erroneous readings may generally be treated under theeffects 
of eidola,interference, mixed shadows, and the shadows or false 
images of a lower structure confusing the outline of the upper 
focal plane of vision; diffraction and chromatism also bear 
their part in the réle of delusive definition. In this way 
black lines, black and white chequers, rows of spherules 
(provisionally offering the signs of cylindrical bodies), and 
crescentic shadows, are all confused into angular, triangular, 
and hexagonal markings, false lights and bright points, the 
resultant focal images of coincident sphericular lenses in con- 
tact. In fact, it appears many of the long-established readings 
of the best microscopes require re-reading. ‘There remains 
work for a generation of new observers. 
It has been considered that the greatest obliquity of illumi- 
nation practicable is necessary to bring out minute structure, 
and therefore the greatest possible aperture (170°) is absolutely 
indispensable to define properly. On this principle all modern 
microscopes are now constructed. A further insight into the 
superior effects of a finer definition will, perhaps, remove this 
obstacle to successful research. A more brilliant, intenser 
display of outline, shadow, and coexistent colouring—a more 
natural picture im relievo, resembling ordinary vision, in which 
the relations of upper and lower planes are better appreciated 
by the eye at one glance—prominences and hollows, eleva- 
tions and depressions—in short, a finer prospect altogether, 
deeper and more finely cut in its tracery, such are the im- 
provements demanded. In low powers the binocular arrange- 
ment has effected much of this. But in the high powers the 
old defects are continually embarrassing the observer. At 
one and the same instant he can only perceive a true sec- 
tional picture, just as we see sectional plates of an engine cut 
through the middle. Continually fresh sections are focussed 
down upon, and the memory must now aid the brain as well 
as the eye to decipher continually dissolving views. 
Erroneous Readings. 
Experiment 14.—It wire gauze be folded and viewed 
against the light at ten inches’ distance, innumerably varied 
