Structure of the Crystalline Lenses of Animals. 195 
on each side of this image one or more highly prismatic images, 
produced by interference, like those of grooved surfaces. 
As the direction of the fibres is necessarily perpendicular to 
the line joining the coloured images, I was enabled to trace 
them to their termination, or points of convergency, when the 
fibres themselves could not be rendered visible by the best 
microscope. But while this new method of observation en- 
abled me to detect all or most of the varieties of fibrous organi- 
zation which exist in the crystalline lenses of animals, it fur- 
nished at the same time a simple and accurate method of de- 
termining the diameter of the fibres at any point of the sphe- 
roid. The distance of any of the coloured images from the 
colourless image was a direct measure of the breadth of the 
fibres; and with the aid of a series of Mr. Barton’s beautiful 
divisions upon steel, from 3125 in an inch to 10,000, which 
he was so kind as to make for me, it was easy to obtain these 
measures without even the trouble of calculation. 
By these means | succeeded in tracing the fibres, or thin 
slender lamin, to two poles diametrically opposite to each 
other, and coinciding with the poles of the spheroidal lens, as 
shown in Plate II. fig. 1. In small lenses, and particularly 
in the lenses of birds, the accurate convergency of the fibres 
to one focus is less distinctly seen; but it is easy to distinguish 
this diffused polarity, as it may be called, from the conver- 
gency of the fibres to points arranged in a straight line, and 
constituting the two septa already mentioned. 
The distribution of the fibres now described is the simplest 
that occurs in the lenses of animals. Every fibre has the 
same length and the same form; and its curvature is, like the 
meridian of a spheroid, without any contrary flexure. 
The perfect flatness of the surfaces of the concentric lamine, 
as indicated by their power of forming a distinct image by re- 
flexion, shows that the fibres which compose them are flat and 
not cylindrical; and when we look through them with a pow- 
erful microscope, this conclusion is amply confirmed by the 
uniform distribution of the refracted light. 
In order to measure the diameter, or rather the breadth of 
the fibres, I detach from the equator of the lens an extremely 
thin lamina, and having placed it above a small aperture in a 
plate of brass, I look through it at a candle, and measure the 
angular distance of the first coloured image from the white or 
central image, taking the centre of the red ray as the point 
from which the measurement is to be taken. When the first 
coloured images on each side of the central image are ex- 
tremely distinct, it will be better to measure the angular di- 
stance of the red parts of their spectra, and to take half that 
Y2 
