PROCEEDINGS OF SOCIETIES. 93 
the fact that the examination of these objects had been chiefly 
confined to their sections, mounted in Canada balsam, which fre- 
quently has the effect of making transparent objects too trans- 
parent. He recommended for this purpose, however, the use of 
Smith and Beck’s semi-paraboloid Lieberkuhn, together with trans- 
mitted light—-cutting off either means of illumination by a slight 
motion of the hand, or using both together; but the readiest 
means of ascertaining their real structure he found to be the exami- 
nation of unmounted and splintered ends of broken spines by 
incident light, a method bringing out details and showing the 
connection of parts in a manner superior to any other. It was 
with diffidence he dissented from Dr. Carpenter’s views in the 
last edition of his work on the microscope; but he begged to pro- 
pose the following as more in accordance with the appearances of 
structure presented by these spines under the microscope. They 
are composed of two substances in outward appearance, though 
chemically perhaps the same, one so perfectly homogeneous and 
transparent when viewed by transmitted or polarized light that 
it cannot be distinguished from the blank field of the miscroscope 
—yet, under incident light, so dark and opaque as to appear black. 
This substance is frequently traversed by winding anastomizing 
channels, which, though only containing air, seem opaque, and 
show as solid by transmitted light, the substance they traverse 
itself being invisible. He had not satisfactorily made out the 
structure of the second substance ; it resembled the pith of plants, 
but it was less regularly cellular, and in some spines assumed 
a fibrous appearance. It is opaque under transmitted, and 
glistening white under incident light. In the following remarks 
he called this opaque, and the first-described transparent sub- 
stance. The general structure of the spines he had examined 
was also twofold in the simpler, as the Amphidotus cordatus, the 
centre portion is hollow ; in the more complicated it is composed 
of the opaque substance perforated along the length of the spine 
by vertical solid tubes of the transparent matter, without any 
definite arrangement. These appear to increase only in length ; 
hence a section at the apex of the spine shows in the centre a 
prolongation of the oldest portion, the thickening of the spine 
arising differently, as subsequently explained. The hollow centre 
of the Amphidotus cordatus is surrounded by a cellular fretwork 
of the transparent matter, while around this is a circle of solid 
ribs or pillars of the same, smooth on the exterior of the spine, 
but within beautifully hollowed out into what the heralds call an 
engrailed outline, the points of which connect it with the inner 
layer of cellular fretwork. This framework is occasionally want- 
ing, and the engrailed points are simply connected with each other 
by a straight inner line of transparent matter. In the more com- 
plicated forms he was not satisfied he had ascertained the real 
structure, but thought it to be as follows :—The tubes of trans- 
parent matter noticed about the central opaque substance, as 
they approached towards the circumference of the first season’s 
