118 



The Microscope. 



and the return journey is duly proceeded with." He 

 adds, " The reasons for, and way in which these 

 motions are effected, are involved in some obscurity."* 

 Professor Smith has suggested that forces operating 

 within the frustule may combine to impel it ; but it 

 would seem that nothing upon the subject has been 

 proved. 



The shape of a diatom cannot always be ascertained 

 by looking at it in one 

 direction, as it is often of 

 some thickness (compara- 

 tively speaking), and its 

 front view very different 

 from that of the side. It 

 is made up of two portions 

 or valves, externally con- 

 vex, and hollow within. 

 No. 15 represents the front and side view of one 

 of those which appear as disks, when their sides 

 only are observed. A dissimilarity of appearance cor- 

 responding to this may be observed on a close 

 inspection of the lowest groups of Gomphonema, in 

 No. 13; their frustules seen in front resemble a figure 

 of 8 rather than a wedge. The valves of the diatoms 

 are sculptured with lines, dots, and other markings, 

 arranged so as to form patterns of exquisite tracery. 

 Some of these are of such extreme minuteness as to be 

 among the most difficult tests for the higher powers 

 of the microscope ; of these Pleurosigma (formerly 

 called Navicula), hippocampus is a well-known 

 * " Diatoms " by Tuffen West, Recreative Science," vol. i., p. CD. 



No. 15. Coscinodiscus in 

 outline a, side view; 

 6, front view. 



