322 EVENINGS AT THE MCEOSCOPE. 



or a structure, from one aspect merely. So many are 

 the chances of illusion, that the student should always 

 seek to view his subject in different aspects, and under 

 varying conditions of light, position, &c. 



It is by making a thin transverse section of a spine 

 — cutting off a slice of it, to speak in homely phrase — 

 that we shall demonstrate the structure, which is 

 very beautiful. This is an operation requiring much 

 delicacy and practice, and implements for the special 

 purpose ; and hence it is best performed by profes- 

 sional persons, who prepare microscopic objects for 

 sale. You may see such a section, however, on this 

 slide ; but I do not know whether the spine belongs to 

 the species we are examining. 



The whole central portion is formed of the sponge- 

 like calcareous matter, which, from the variously 

 reflected and refracted rays of light, appears nearly 

 opaque, and of a bluish colour by transmitted light. 

 This structure sends forth radiating points (making 

 longitudinal ridges, of course, in the perfect spine;) 

 and it is the opacity of these points (or ridges) which 

 reach the circumference, that gives to the spine the 

 appearance of being fluted. Indeed it would be fluted 

 if this were the entire structure ; but the open space 

 left between these projecting radii is filled with the 

 solid glassy matter, having, as we see, a convex sur- 

 face. This, however, from its perfect transparency, 

 is not seen when we look at the side of the spine, the 

 eye going down to the bottom of the interspace. The 

 spine is, in fact, a fluted column of spongy glass, with 

 the grooves filled with solid glass. 



We have not yet seen, however, the beautiful 



