508 Transactions of the Society, 



which are scattered all sorts of fragments of the diatom sections, 

 which the observer may be able to define with reference to their 

 previous position, but which are utterly useless for the study of real 

 details of structure. The unavoidable proximity of finger and 

 face during the cutting is injurious, because the immediate sur- 

 rounding atmosphere of the operator always contains a quantity 

 of moisture which causes the dehcate chips of gum immediately 

 to adhere. In pihng-up the sections we should most carefully 

 avoid every increase of moisture, and during the operation 

 the breathing should be suppressed. If after discharging the 

 sections from the knife we do not intend to put on the cover-glass 

 at once, then the slide with the chips should only be lifted up when 

 in sunlight and under protection from dust. As a rule, time 

 should not here be lost. It is advisable before putting on the 

 cover-glass to touch two corners, which first come in contact with 

 the slide, with small drops of balsam, so that during the lowering 

 of the cover-glass it does not slip out of place and so grind up the 

 very brittle chips of gum. The lowering of the cover-glass should 

 be done slowly and steadily. If let fall, the puff of air will blow 

 away the chips. If the chips, on account of too little moisture in 

 the atmosphere, are curled up, the cover-glass will flatten them 

 out, the gum on the edges may spht, but the centre becomes 

 flat and is often very useful. In general, however, it is recom- 

 mended, on the days when these difficulties occur, to postpone the 

 operation. Once for all, I state that flat sections are always the 

 most instructive. After putting on the cover-glass a proportion- 

 ately small drop of thin fluid-balsam is added on the edge — only so 

 much that the sections are imbedded in it. After a few days the 

 vacant spaces are filled up by fresh drops of balsam ; a derangement 

 of the previously filled-up sections is no longer to be feared. 



Series Prejoarations. — With Pinnularia the making of series 

 sections is almost a necessity, especially for the longitudinal sections. 

 My procedure is as follows, though I am well aware it is capable 

 of improvement, I cut off No. 1, leaving it on the edge of the 

 razor ; then I take a second section a little further off, and so 

 on, until five are on the edge. These I transfer to the slide 

 with a needle, and as nearly as possible in a straight line. This 

 is repeated with the other five sections which form the second 

 line. Then I take a new slide, and thus get decades of sections 

 from a bundle previously prepared. A disadvantage is that some- 

 times the sections do not remain on the edge ; by falling upon the 

 table th^y are lost. Also that not seldom one cuts a gum-chip 

 as a numbered section, which on further examination proves never 

 to have touched the bundle of frustules. By disregarding these 

 very troublesome mishaps, the series-section method according to 

 my view gives the most beautiful results obtainable with such 



