154 THE MICROSCOPE. 



mens are mixed with much foreign matter, in the form of minute 

 particles of mud or sand, which impair their value, and interfere with 

 observation, especially with the higher powers of his instrument. 

 These substances the students may remove in various ways : by re- 

 peated washings in pure water, and at the same time profiting by the 

 various specific gravities of the Diatorus and the intermixed substances, 

 to secure their separation ; but, more particularly, by availing himself 

 of the tendency which the Diatomacese generally have to make their 

 way towards the light. This affords an easy mode of separating and 

 procuring them in a tolerably clean state ; all that is necessary being 

 to place the gathering which contains them in a shallow vessel, and 

 leave them undisturbed for a sufficient length of time in the sun- 

 light, and then carefully remove them from the surface of the mud or 

 water. 



The simplest method of preserving the specimens, and the one 

 most generally useful to the scientific observer, is simply to dry them 

 upon small portions of talc, which can at any time be placed under the 

 microscope, and examined without further preparation ; and this mode 

 possesses one great advantage, that is, that the specimens can be 

 submitted without further preparation to a heat sufficient to remove 

 all the cell-contents and softer parts, leaving the siliceous epiderm in a 

 transparent state." 



In the vicinity of Hull many very interesting varieties of Diato- 

 maceaB have been found, the beauty of the varied forms of which are 

 such as to delight the microscopist ; and, at the same time, some of 

 them are highly useful, as forming that class of test objects which are 

 best calculated of all others for determining the excellence and powers 

 of our object-glasses. It has been shown by Mr. Sollitt that the mark- 

 ings on some of the shells were so fine as to range between the 30,000th 

 and 60,000th of an inch ; the Pleurosigma strigilis having the strongest 

 markings, and the Navicula acus the finest. 



DESCRIPTION OF PLATE II. 



1. Shell of Arachnoidiscus. 2. Shell of Actinocyclus (Bermuda). 3. Shell of 

 Cocconeis (Algoa Bay). 4. Shell of Coscinodiscus (Bermuda). 5. Shell of Isthmia 

 enervis. 6. Zygoceros rhombus. 7. Campylodiscus clypeus. 8. Amphitetras. 9. 

 Gallionella sulcata. 10. Triceratium, found in Thames' mud. 11. Gomphonema ge- 

 minatum, with their stalk-like attachments. 12. Dictyocha fibula. 13. Eunotia. 

 14. Cocconema. 15. Fragilaria pectinalis. 16. Meridion circulare. 17. Diatoma 

 flocculosum. 



