208 ON AN EASY METHOD OF VIEWING DIATOMACE/E. 



covered with the thinnest possible glass, and the slide should 

 be perfectly clean and free from damp, otherwise the field 

 will have a milky appearance. In using the spotted lens, 

 every part of the microscope must also be perfectly clean and 

 free from dust ; the concave mirror should always be used with 

 it. When due precautions are taken, points of structure can 

 with its aid be easily made out not seen by the ordinary 

 methods of illumination. 



What may be the exact nature of the striae is not easy to 

 determine. " Whenever," says Mr. Quekett, "these infu- 

 soria are viewed under the most favourable illumination, 

 either from a white cloud or a lamp with direct light, and a 

 magnifying power of at least 1,200 diameters, the lines are 

 all shown to be dots or elevations from the surface." * The 

 Rev. W. Smith t considers the true character of these mark- 

 ings to have been mistaken : " some observers having consi- 

 dered those appearances to arise from a series of perforations, 

 others from rows of beads or minute elevations." From the 

 close manner in which the striae are arranged, their resolution 

 is amongst the most difficult tasks in microscopy. After 

 having given to the subject no little care and attention, with 

 an eighth object-glass made by Messrs. Powell and Lealand 

 (the beautiful defining powers of which it is impossible to 

 estimate too highly), and a very deep eye-piece, I have little 

 hesitation in now concluding with Mr. Smith that the form of 

 these markings is hexagonal. 



In speaking of these curious structures it will be seen that 

 the division of Mr. Smith has been followed ; and that the 

 genus Naviciila of Kiitzing is divided into Navicula, distin- 

 guished by the delicacy of the striae, and their moniliform 

 character ; Pinnularia, from the striae, owing to the confluent 

 nature of the cellular structure of its epiderm, having the 

 appearance of distinct costae ; and Pleurosigma from the 

 characteristic curve of its beautiful frustules. 



* Quekett on the Microscope, 2nd Edit., p. 475. 

 t Smith, vol, i,, pp. 61, 62. 



