30 
colour; while 
Echinus lividus was more solid, the pith-like substance 
passing into the solid glassy radiating plates through portions 
consisting of this glassy matter, perforated by anastomising 
channels. 
Mr. Hurst was not able to say whether the pithy and glassy 
substances are distinct or not ; but while the cellular matter 
leaves no trace after the prolonged action of vinegar, the 
transparent glassy substance, as well as the exterior of the 
spine, appears to be enveloped by a membrane, resisting the 
action of vinegar, which curiously converts this solid, 
opaque, hard, and brittle spine into a transparent, flexible 
body, retaining its original form. 
By using direct sunlight and a semi-paraboloid condenser, 
the glassy matter could be distinctly seen through, even 
when viewed as an opaque object, and the arrangement of 
the cellular matter ascertained. It is this transparent 
substance which is tinged with the beautiful purple hue so 
well known to microscopists. 
Mr. Hurst expressed his disappointment that the use of 
polarized light in these observations had led to no result, 
and thought its value had been over-estimated. 
