506 Transactions of ike Society. 



proximity of the face and the hands, the aqueous contents of such 

 strongly hygroscopic substances are subject to an uncontrollable 

 change, so that we must work with pure gum arable and in the 

 summer. 



Slight differences are caused by the weather or by the position 

 of the sun. Sections of the coarser diatoms which are not required 

 to be of extreme thinness are best made with a clouded sky or in 

 rainy weather. Sections of Pleurosigima succeed best when the sun 

 shines directly into the laboratory without striking upon the Micro- 

 scope. I can hardly too strongly dwell upon the necessity of 

 entering upon such experimental investigations under the best 

 physical conditions, absence of vibration, noise, &c. 



Placing the Frustule. — By my earlier method sections in 

 different directions were obtained hap-hazard because the frustules 

 were lying pell-mell in the gum. I have now improved the method 

 as follows : — I take a number of frustules like a bundle of rods and 

 cut sections in the exact transverse or longitudinal direction. With 

 this important but difficult method every one must become familiar 

 if he intends to check the results (hereafter described) which I 

 obtained with Pinnularia. 



(1) If we wish to examine uninjured specimens, the diatoms are 

 first stained, usually by picro-carmine ; they are then put in absolute 

 alcohol. A glass slip is coated with collodion, and allowed to set. 

 To avoid peeling when dry, the collodion must not be too thick. A 

 drop of thick gum is then put on. A cluster of diatoms is taken 

 direct from the alcohol with forceps and placed in the gum. In 

 consequence of the current set up, the diatoms immediately distribute 

 themselves equally through the gum. As soon as the edge of the 

 gum begins to harden, one frustule after the other is drawn to the 

 edge by a very fine needle, where, with proper manipulation, they 

 can be piled up like a bundle of rods. All those which interfere with 

 this piling up should be removed. Owing to the staining, the 

 frustules can be readily seen on the transparent ground. As soon 

 as the edge dries, a drop of fluid gum is added by a needle, and this 

 process is repeated until the solid layer of gum has reached such a 

 thickness that a displacement of the frustules need be no longer 

 feared ; a small patch of collodion is then put on. The preparation 

 is now cut out by four cross-cuts and carefully removed from the 

 glass ; the bundle of diatoms in gum being contained between two 

 films of collodion. It is advisable to make a drawing with a high 

 power to show the position of the individual frustules and aid in 

 the identification of the sections. The preparation is then put 

 upon a nearly dry flat drop of gum on a piece of cardboard, and the 

 base and the edges are made to adhere, if necessary, by a few drops 

 of water, and by the addition of minute drops of gum it is so im- 

 bedded that at last it is entirely surrounded. This must be carried 



