280 EVENINGS AT THE MICROSCOPE. 



laterally, the appearance is such that we cannot but firmly 

 believe, that it is grooved throughout with straight and 

 deep longitudinal furrows. But if we break off the same 

 spine transversely, and so exhibit it that the broken end 

 shall be presented to the eye, we perceive that there are 

 no grooves ; but that the points in the circumference, 

 which seemed to be the summits of the ridges, which are 

 very narrow, are really lower than the intermediate spaces, 

 which we supposed to be the grooves, and that the surface 

 of these spaces is really convex in a slight degree. 



The explanation of these contradictory appearances is 

 easily given. Meanwhile, however, they read an important 

 lesson to the inexperienced microscopist, not to decide too 

 hastily on the character of a surface 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 beau- 

 tiful. This is an operation requiring much delicacy and 

 practice, and implements for the special purpose ; and 

 hence it is best performed by professional 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 



