312 



THE MICROSCOPE. 



more we perceive that, in many species at least, the shape 

 or outline is subject to endless variations." 1 



I recommend microscopists to conduct their observa- 

 tions of these and similar bodies in very shallow cells, say 

 of from l-50th to l-100th of an inch deep, covered with 

 glass of from 1-1 50th to l-250th of an inch thick. The 

 objective must be a l-4th or 1-Sth, with a good eye-piece, 

 and careful illumination Rainey's moderator with a 

 Gillett's condenser or parabolic reflector. The examina- 

 tion should be conducted during very bright weather, or 

 by sunshine. 



The markings on 

 the Diatomacece are 

 best seen when mount- 

 ed dry : they are usu- 

 ally mounted in Ca- 

 nadian balsam, or in 

 weak spirit arid water. 

 Before quitting these 

 interesting objects, we 

 shall notice a few of 

 the commoner forms, 

 reserving our remarks 

 upon other species 

 until we come to fossil 

 Infusoria. 



Pleurosigmata Na- 

 mculce. Navicula,the 

 Latin for ship, has 

 been applied to these 

 little creatures from 

 their resemblance in 

 form to a ship or boat. 

 In the catalogue of the 

 microscopist there are 

 upwards of twenty- 

 tour different species 

 named, fourteen of which may be found alive, fig. 181. 

 It was in 1841 that Messrs. Harrison and Mr. Sollitt dis- 



* Dr. Gregory, "On a remarkable group of Diatomaceous Forms." Quarter fy 

 Journal of Microscopical Science. 



Fig. 181 



1, Pleurosigma ftttenuatum. 2, Pleurosiyma an- 

 gulatum, magnified 250 diameters. 3, Pleuro- 

 sigma Spencerii, magnified 350 diameters.. 



