6 
black ring outline of refracting spherules and black borders 
of cylindrical fibres. In innumerable instances the only pos- 
sibility of distinguishing the molecules of organized particles 
depends upon shadow. It will be seen, further on, that the 
visibility of this shadow is entirely governed by aperture and 
refractive index in the observed substance. ‘The human eye, 
in natural vision, sees an object 10 inches’ distance under a 
less aperture than 2°, and for distant objects it becomes less 
than a second of arc. The gaseous lenticle just described 
presents to ordinary vision a delicate jet black ring, and 
glass sphericles and cylindricles give a black boundary. A 
fundamental defect of excessive aperture is the disappearance 
of these invaluable characters of minute sphericles and fibres 
capable of refracting light. An extraordinary variety of 
objects, critically examined, develop a new order of ap- 
pearances under reduced aperture totally invisible with the 
same uniform aperture generally employed by the observer, 
yet the advantage of changing telescopic aperture has been 
long appreciated and acted upon. 
It also appears that very large aperture is incompatible 
with any considerable depth of focal vision or with focal per- 
spective (if a new term may be allowed). Intense definition 
combined with a deeper perspective are qualities which now 
form the great desideratum of histological research. 
Under high power focal perspective is almost inappre- 
ciably shallow, and until this can be deepened only a mere 
skeleton section or plane of the object can appear in the field 
of the microscope at once. As no attempts have been made 
to measure this visual range for different powers, it may 
not be uninteresting to record an experiment made in the 
spring of 1869 with a very small pencil of parallel solar rays 
illuminating obliquely some beautiful spherules (of a scale 
of “ azure blue’’), lying in close contact, =,1,,th of an inch 
in diameter. With the highest power used (4000), the point 
of distinct vision appeared palpably to travel through the 
beading, and by estimation (as near as could be done) the 
visual perspective extended only one fourth through the bead, 
OY +seocuth of an inch. In other words, nothing above or 
beneath a plate of that thickness was visible with the same 
focus. The focal plate, if I may so name it, may be roughly 
estimated, from the results of several experiments, as 
follows : 
1 It has already been observed angular aperture of microscopes varies 
from 15° to 170°, unlike the telescopic aperture measured by inches. 
