32 RAMBLES AND REVERIES. 



edges. This flinty covering is perforated, in order 

 to allow of communication with the outer world. 



The minuteness of these plants is marvellous 

 when we bear in mind how perfect they are in every 

 detail of their form and markings. The giants 

 amongst them are not more than about one-fiftieth 

 of an inch in length, while the majority are less 

 than one-thousandth of an inch. My friend, Mr. 

 Isaac Robinson, of Hertford, who has made these 

 organisms a special study, relates that one of his 

 slides which contains the frustules from a single 

 drop of water has upon it no less than 200,000 

 frustules, and another reaches the astonishing num- 

 ber of 430,000. And the water itself has to be 

 taken into account in attempting to form an idea of 

 the size of these infinitesimal inhabitants of that 

 small world. If the bulk of the drop of water had 

 consisted exclusively of frustules, there would have 

 been, in the latter case, upwards of ten millions. 

 How much truer it appears now than Shakespeare 

 ever imagined when he made Hamlet say, " There 

 are more things in heaven and earth, Horatio, than 

 are dreamt of in our philosophy ! " 



The utmost diversity of form is met with amongst 

 these minute plants. They are boat-shaped, tri- 

 angular, square, crescent-like, filamentous, and sea- 

 weed shaped ; in fact, there is scarcely any conceiv- 

 able pattern which does not find its counterpart 

 amongst these extraordinary objects. 



The mounted frustules of Diatoms are extensively 

 used as tests for the determination of the quality 

 of microscopic lenses. An illustration of one of 



