6 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 60. 



the investigator wishes to have his diatoms as free as possible from 

 extraneous matter. 



The thoroughly washed and cleaned material is now put up in 

 bottles in 35 per cent alcohol and properly labeled. 



No attempt will here be made to describe the subsequent mounting 

 of clean diatoms. This must be left to the wishes of the investigator, 

 dependent upon whether his purpose is to make strewings of the 

 diatoms or to pick the individuals out and mount them separately. If 

 the latter and of course the better method is used, it may be well to 

 add one or two suggestions. The writer uses special microscope slides 

 for picking out diatoms. The slides selected are thick, about 2 mm. 

 Eleven lines are drawn across the slide near the middle, about 3 mm. 

 apart with a writing diamond, and a median line is drawn across these 

 11 lines bisecting them, the lines thereby producing 20 spaces. Fig- 

 ures from 1 to 10 are written near the margin of the slide, numbering 

 the 10 cross divisions. One has thereby 20 spaces, upper and lower 

 No. 1, upper and lower No. 2, etc., and in picking diatoms he can 

 proceed in an orderly way from one side to the other, back and forth 

 through the spaces marked by the cross lines ; and when a diatom is 

 found and removed for mounting, he is able with this slide to go back 

 to the exact place where he left off by remembering the part of the 

 subdivision wdiere the selection was made. He therefore avoids going 

 over the same ground twice or missing part of the material on the 

 slide. 



In making a strewing from which to pick individual diatoms a 

 couple of drops of the clean material well shaken up is placed upon 

 the slide, spread out over the lines by tilting the slide, and the water 

 and alcohol evaporated over a spirit lamp. The diatoms should lie 

 evenly strewn on the slide and be perfectly dry. If the cleaning has 

 been properly done and distilled water and absolute alcohol have 

 been used for the 35 per cent solution, the diatoms should be loose 

 upon the glass and can be picked up with the greatest ease. The only 

 wholly satisfactory way of accomplishing this selecting and mount- 

 ing of individual diatoms is by means of the apparatus known as the 

 mechanical finger. The method of using this delicate instrument 

 can not be taught by description. It must be demonstrated, and is 

 even then a part of diatom technic which requires long practice. But 

 the result obtained is so superior to diatom specimens in a strewn slide 

 that the learning of this process is strongly recommended. A strewn 

 slide bearing the name of any specific diatom contains many others, 

 and some may be very similar in general appearance. How is the 

 student to pick out the true type from this mass of material, when 

 he probabl}?^ is looking for an example of the type in order to know 

 how it looks ? But if each diatom is mounted separately, it becomes 



