594 
MR, B. T. LOWNE ON THE MODIFICATIONS OF 
glass threads, drawn from glass rod, in the interior of the capillary tube ; these were 
as nearly as possible the same length as the tube, and measured yinro of an inch in 
diameter. The end of each of these fine rods appeared as a small bright disk in the 
deep black lumen of the tube, and when the light was shut off from the rest of the 
fi old reminded me of the appearance of the disk of a planet seen through a telescope, 
although the illumination was not by any means powerful, but was obtained from an 
ordinary gas burner ten feet from the microscope. When the focus of the microscope 
was altered so that the ends of the rods lay beyond it, the circles of light enlarged, 
showing that the rays left the rod in a divergent direction. 
The same phenomena were observed when the diameter of both the rods and the 
containing tube was somewhat increased, but they were not so brilliant. In some 
cases when the ends of the rods were beyond the focus of the object glass, the central 
spot of light was surrounded by grey rings from interference. When the extremities 
of the rods were lenticular, as well as when the lower end of the rod was enlarged by 
fusing it into a globule of glass, or when it was drawn into a cone, the same phe- 
nomena occurred ; so the brilliancy of the upper extremity of the rods was increased 
when the light, falling on the lower extremity, was convergent, so long as the axis of 
the ray was in the same direction as the axis of the rod, although oblique pencils 
produced only very feeble illumination of the upper extremity of the rod, even when 
the obliquity was only slight — at most, three or four degrees from the axis of the rod. * 
It appears pretty evident that the appearances described are due in some way to 
total internal reflection. 
In order to estimate the effect of the pigment I used glass rods, covered, except at 
their ends, with a layer of black varnish ; and I found that even with rods -g^Q-th 0 f an 
inch in diameter, and only half an inch long, it was very difficult to transmit any light 
at all, unless the rays were absolutely parallel with the axis of the rod. With 
longer lengths of rod, or rods of smaller diameter, no light was transmitted, and the 
ends of the rods appeared quite black. 
From these facts I think the inner extremities of the fine rod-like structures of 
highly refractive material which extend from the cornea, or from the inner end of the 
chamber into the deeper structures of the eye, may be regarded in the light of 
luminous points, illuminated by the light of the central pencil transmitted through 
the lens, and having as their function the excitation of the nerve ending in which 
they are embedded. 
The focus of the lenticular facet in all the insects which I have examined lies 
considerably deeper than the outer extremity of the rhabdion in the true compound 
eye, and much below the surface of the rod-like recipient structures in the micro- 
rhabdic eye, so that for objects removed only the tenth of an inch or less from the 
* Oscae Schmidt lias since recorded some similar experiments with glass rods in Kollikee, ‘ Zeitsch. fur 
Wissenscli. Zoo.’, Bd. 30. Beiblatt. 
