348 



1 HE GUIDE TO NATURE 



practice, which is necessary in this 

 department of science as in any other. 

 A little clever adjustment will illumin- 

 ate our field of view admirably. There 

 it is, and perhaps not one fellow in a 

 hundred, not a microscopist, would be 

 likely to guess correctly its origin. A 

 disc, sculptured and fretted with 

 wonderful work, with ornamentation 

 more elaborate and more beautiful 

 than that of any rose window in the 

 cathedrals of the Continent. What is 

 it, and what is its real size? 



Well, it is the flinty skeleton of a 

 minute water plant, and as the tiny 

 atom is practically indestructible ) this 

 very individual may possibly have ten- 

 anted the earth about the time of Abra- 

 ham. Who shall say? It may be in ex- 

 istence another four thousand years for 

 any mutability which is in itself. As 

 for its size, we measure and learn its 

 diameter is one-sixtieth of an inch, so 

 that some sixty of them can be easily 

 ranged in that space with an interval 

 between each. Who would deny it is 

 His handiwork Who not only made the 

 heavens but moulds a dewdrop? Yet 

 an onlooker exclaimed. "What a waste 

 of beautiful work, for the chances must 



have been a million to one that no one 

 ever saw it !" Without doubt they 

 •were not made primarily for the delec- 

 tation of our eyesight. Each line of 

 silex, that surface, has its function and 

 plays its part in the life economy of the 

 plant, no student of nature will hesitate 

 to affirm ; but what that part is, how 

 it is performed is a mystery, perhaps 

 forever, even from such inquisitive and 

 eager eyes as yours and mine. We ex- 

 amine one form, but they are legion! 

 Amongst other genera are some called 

 A^ini/a-boat-shaped diatoms ; Arach- 

 noidisc us-spider webbed ; Coscinodiscus- 

 circular shaped ; Pleurosigma-resem- 

 bling the letter S of slight and varying 

 degrees of curvature. This last type 

 has many species, varying in number 

 and thickness of the markings or striae ; 

 and very largely used as "test objects" 

 to estimate the quality of your objec- 

 tives and eyepieces, your skill in hand- 

 ling your condenser, and machine as 

 well as the illumination. Of their suit- 

 ability I cannot chat or I would never 

 go to bed to-night. Where doctors 

 disagree, who shall decide? The great 

 "fans" of the micro-world differ as the 

 poles from the antipodes, so you and 



~ . ■ 





O 



© 



A TYPE SLIDE TO SHOW VARIETY OF FORMS AND PATTERNS OF DIATOMS. 

 All this group occupied a space on the slide of not much more than the head of a pin. 



