510 THE MICROSCOPE. 



in St. Catherine's Island at Tenby, might reap a harvest 

 which would afford amusement and instruction for many 

 weeks. These caverns are so rich in zoophytes and 

 sponges, that they are literally roofed with the Laomedece, 

 Grantice, and their allies ; whilst the elegant TuhularioB 

 alford an ornament to the shallow pools on the floor ; and 

 the walls are wreathed with the pink, yellow, green, and 

 purple Actiidce. 



When these objects are examined by polarised light, 

 most interesting results are produced. For this purpose, 

 let a piece of selenite be placed on the stage of the micro- 

 scope, and the polarising prisms arranged so that the ray 

 transmitted is absorbed by the analyser. If a specimen of 

 Sertularia operculata be placed on the selenite stage, and 

 examined with a two-inch object-glass, the central stem is 

 shown to be a continuous tube, assuming a pink tint 

 throughout its whole extent. The cells appear violet in 

 colour ; their pointed orifices are seen much more distinctly 

 than when viewed with common light. The vesicles are 

 paler than the rest of the object ; and their lids, which so 

 remarkably resemble the operculum of the theca of a 

 moss, are beautifully distinct, being of an orange-yellowish 

 colour. This zoophyte is often covered with minute 

 bivalve shells, distinguished by the naked eye from the 

 vesicles only by their circular form ; and these, when pre- 

 sent, add much to the beauty of the specimen, presenting 

 a striated structure, and becoming illuminated with most 

 beautitul colours. 



