564 THE SENSES AND SENSORY ORGANS. 
Schmidt), the crystalline cones are drawn out into long fine 
threads, often curved more or less in their long axes. Schmidt 
[220], and myself independently [219], made experiments on 
glass rods, models on a large scale of the cones of the Phroni- 
mide. Schmidt found that when the convex conical end of 
the rod is illuminated the light is emitted in a divergent pencil 
from the other end of the glass thread. In my experiments 
I used rods 45 of an inch in diameter and an inch in length; 
these were enclosed in a capillary glass tube filled with 
water, and whether curved or straight they transmitted light, 
although so little passed through the lumen of the tube that 
it appeared black when seen with a half-inch objective. The 
capillary tube was placed in the axis of the microscope so 
that the end nearest the object-glass was level with the stage ; 
the lower end was illuminated directly by the lamp. When 
the focus of the microscope was altered, so that the ends of 
the rods were beyond it, the discs of light enlarged, showing 
that the emitted light was divergent. 
The light which enters a cylinder with divergent rays after 
any number of total reflections, emerges from it with divergent 
rays and the angle of divergence remains the same. When 
the cylinder is also a refractive cylinder it will have a focus 
like a lens, and this focus will be a recurring function of its 
length (see p. 572); so that whether the thread conducting 
the light-rays is a simple cylinder, with a lens situated near or 
at its extremity, or whether it is a refractive cylinder, in 
Exner’s sense, a small image is capable of being formed 
beyond its emergent surface. 
The conditions necessary for,the formation of such an image 
are that the conducting-rod shall be a cylinder, and not a cone, 
as total reflection from the inner surface of a cone produces 
divergent pencils at its apex, and the internal surface of the 
rod must be capable of reflecting the incident light. 
I have found that small glass threads do not possess this 
power when the outer surface is blackened with soot, or even 
with black varnish. The blackened surface absorbs and does not 
reflect incident pencils; even a narrow ring of pigment prevents 
