226 
NOTES AND MEMORANDA. 
light passing into the rod, except very ohliqne rays, would he totally 
reflected, without any change of phase in the undulations, at the 
surface of the glass, whilst all except the axial rays would be very 
much enfeebled by numerous reflexions and interference from the 
different lengths of the paths of the rays. In his opinion, threads of 
a highly refractive character immersed in a medium of a less refractive 
index, when less than 5 of an inch in diameter, would destroy the 
efl“ect of rays of only very small obliquity by interference. 
In order to determine the effect of the pigment, he covered the 
exterior of some glass rods of 5^^ of an inch in diameter with black 
varnish, and then found it impossible to transmit any rays of even 
the slightest obliquity through half an inch of such a rod. 
From these facts he thinks it may be concluded that it is probable 
that the highly refractive structures may be regarded in the light of 
luminous points, which serve as stimuli in exciting the recipient 
protoplasm in which their ends are imbedded. 
The focus of the facet when this is lenticular, in all the insects 
examined, is situated considerably deeper than the outer end of the 
rhabdion and below the surface of the rod cells in the microrhabdic 
eye, so that even for objects as close as of an inch to the cornea, 
we have to deal with convergent rays, and not with a focal point. 
This indicates some mode of nerve stimulation other than the union 
of homocentric pencils, in a point beneath the compound cornea in 
relation with the recipient elements. Considering the small size of 
the parts, it is quite possible that we must look to the phenomena 
of interference for the explanation ; at least, they must play an 
important part in the phenomenon. 
Whatever may be the manner in which vision is accomplished, the 
size of the corneal facets and the general curvature of the cornea 
render the theory of J. Muller highly probable. It is true that 
ClaparMe has expressed the reverse opinion, but he has done so 
on insufficient data. According to his calculation, a bee should be 
unable to distinguish objects of less than eight inches in diameter at 
a distance of twenty feet from it. This calculation is based on the 
idea that the acuity of vision in this insect is the same in all parts of 
the field of vision, and that the general surface of the common cornea 
is approximately a segment of a sphere. This is not the case, for the 
angles subtended by the adjacent facets in the centre of the cornea, 
which is considerably flattened, is not more than half a degree at the 
most ; so that on J. Muller’s theory, supposing each facet to give rise 
to only a single luminous impression, the bee should be able to dis- 
tinguish objects of about two inches in diameter at a distance of 
twenty feet, an acuity of vision quite equal to account for all the 
phenomena of vision in bees. 
The curvature of the cornea of a number of insects was measured, 
with a view to determining the angles made by the lines of vision 
drawn from the centre of adjacent facets. This is done in the fol- 
lowing manner : — A magnified image of the cornea is thrown on a 
sheet of white paper, by means of a microscope and camera lucida, 
and the curve of its profile drawn ; in this way the principal meridians 
