350 The Microscope. 



paraffin, through which the lines required are to be ruled with a 

 fine pointed needle. If then some finely powdered fluor-spar is 

 put in a leaden or a platinum cup with some concentrated sul- 

 phuric acid, and the cover is placed over it, the lines will be 

 acted upon by the hydrofluoric acid fumes and the glass will 

 be etched accordingly. The cover glass should then be de- 

 tached from the slip, cleaned and mounted on a circle of card- 

 board, but to a size to fit into the eye piece. In this way micro- 

 scopical drawing becomes exceedingl}'- easy, especially when the 

 sectional paper is used. 



A METHOD OF EQUALIZING THE THICKNESS OP SLIPS FOR USE 



WITH AN OIL IMMERSION CONDENSER.* — It is nccessary that an 

 oil immersion condenser should have a fairly long focus, other- 

 wise it would be of no use if the slip happened to be rather 

 thick. If the slip is thin it will be found impossible to keep 

 the oil contact when the condenser is in focus, unless you in- 

 crease the thickness of the slip by uniting a thick cover glass to 

 the back by oil. It will be found very difficult to do this with- 

 out oiling the stage when the microscope is inclined. The oil 

 between the condenser and the cover glass is sure to unite with 

 that between the cover glass and slip, and then the cover glass 

 falls, upsetting the whole arrangement. I have found the fol- 

 lowing plan to answer admirably : A piece of glass one inch 

 square, upon one side of which, close to the edge, a strip ^ inch 

 broad is fastened by shellac, is oiled to the back of the slip; 

 the ledge hooking over the edge of the slide keeps it from 

 slipping down. — jE. M. Nelson. 



Structure of diatoms. — It having occurred to Mr. C. H. 

 Gill* that if the markings were cavities they must have definite 

 functions, and must be in connection with the interior cavity of 

 the Diatom at least, probably filled with the living plasma of 

 the organism, and further be in communication with the out- 

 side by other perforatiojis in their exterior face. Then if the 

 markings were cavities, they could be filled with foreign bodies, 

 and if those bodies were opaque or deeply colored it would be 

 easy to recognize their presence. After trying many different 



*Journ. Q. M. C, 



