HARD WICKE 'S S CIENCE ■ G O SSI P. 



219 



drop of material can be wiped off, and the selected 

 diatom pushed to the centre of the slide. Some 

 species of diatoms are too transparent to be seen 

 wneii In. water. I then allow them to become dry, 

 and place the slide (bottoin >..p,„n,vUi under the com- 

 pound microscope, and examine it with a | objective, 

 and if any form should be observed that I wish to 

 select, I place a small ink-dot just above it ; this 

 enables me to detect it with the simple lens. The 

 best instrument for picking out is a " feeler" from a 

 hare or rabbit. As these gradually taper to a point, 

 an opportunity is afforded of obtaining any stiffness 

 that may be found desirable. It should be mounted 

 on a light handle (like those used for camel-hair 

 pencils) by slipping over it the quill of a small wing- 

 feather from a pigeon or partridge ; the hair can be 

 adjusted by drawing it up or down, and when found 

 to be satisfactory, press the handle tightly into the 

 quill and cut off the superfluous hair. (Captan Lang, in 

 M. M. y., December 1st, 1870, p. 308, recommends 

 a badger hair, but I do not find it stiff enough to push 

 the diatoms out of the drop, or detach them, if dried, 

 on the slide.) The learner will probably find to his 

 gi-eat annoyance that the diatoms are disturbed by the 

 application of the balsam and heat, and exhibit a 

 tendency to float in it, and worst of all, when the 

 cover is put on, a large proportion of them escape 

 with the squeezed-out balsam. This may be counter- 

 acted in two ways. First (my plan), let the drop of 

 diatoms be as small as possible and very full, taking 

 care that the turpentine does not disturb them ; the 

 quantity of balsam should be just enough to fill the 

 space occupied by the cover-glass when pressed down. 

 If this is carefully done, the diatoms will be evenly 

 distributed, and the slide will not require cleaning. 

 The second plan is to place a little gum in the last 

 washing, or a better plan is to make a solution of it 

 in distilled water (i^ grains of gum arable to I grain 

 of water), put a drop of this on the cover or slide, and 

 then drop a little of the diatom material in it. 

 When dry, the diatoms will be found firmly fixed to 

 the glass. The gum-water can also be used for fixing 

 selected specimens. The tyro will sometimes find 

 to his intense disgust that his carefully selected 

 specimens have become smashed ; this is generally 

 caused by too much pressure on the cover-glass. In 

 order to avoid this risk, a little cell, about ^o' o^ 

 an inch in diameter, should be spun on the slide : the 

 gmii and whiting before alluded to will be found very 

 useful for this purpose ; or a little lamp-black or ver- 

 milion may be mixed with it, and if the cell is neatly 

 made, it wiU not injure the appearance of the slide, 

 and it has the advantage of enabling the observer to 

 find the objects without trouble. I, however, give 

 the preference, myself, to cells made of thin glass, of 

 the same diameter as the cover ; these are not difiicult 

 to make, and when the cover-glass is very thin, it 

 sometimes prevents a smash when the objective is 

 focussed upon it. The cells are made in the follow- 



ing manner : — A piece of brass, the size of an ordinary 

 slide, and -^ of an inch in thickness, is perforated in 

 the centre (the size of the hole might be -f^ of an inch 

 in diameter) ; a disc of thin glass is cemented over it 

 with shell lac. When cold, the centre may be easily 

 Ki<,,.i.„ri niit with a small round file or steel broach. 

 Re-heat the brass ana sup ,=.«• +1,^ ^pn \^<^q some 

 methylated 'spirit, which will speedily dissolve off the 

 lac ; by using five or six brass plates, a stock of cells 

 can soon be made. In order to attach them to the 

 slides, I spread a little balsam upon it and harden the 

 balsam, place the cell in position, re-heat the slide 

 and press it down, drop a little more balsam into the 

 cell, which should be hardened, but in a less degree 

 than for fixing the cell. The diatoms already placed 

 on the cover and balsamed may be now finally placed 

 over the cell, and the cover pressed without risk. 



In using gum, care must be taken to avoid using 

 more than is absolutely necessarj' for fixing the 

 diatoms, as it injures their sharpness ; in fact, I have 

 long given up using it, and prefer the following 

 method when the diatoms are not too delicate. We 

 will suppose them picked out and clean. I put a 

 drop of turpentine upon them, and then some balsam, 

 which should be thin ; on another slide I place 

 some more balsam. I now slightly warm the slide 

 containing the diatoms, and with a bristle rather stiffer 

 than that used for picking out of water, I take them 

 out of the balsam and transfer them to the balsam-slide 

 No. 2. When they are all removed, I pi'oceed to 

 arrange them, which is easily done if the balsam is 

 kept fluid by heating. The diatoms may be pressed 

 down with the bristle, and as the balsam hardens 

 they retain the position in which they are placed. 

 This method would not answer for slides like Moller's 

 Typen Platte : these ai-e arranged and fixed to the 

 cover with gum. Captain Lang approves highly of 

 the following plan of his friend Captain Haig : — 

 He smears the slide or cover with a little glycerine, 

 to which a little gum has been added ; into this the 

 diatoms are placed and afterwards anranged. Glyce- 

 rine has this advantage over water, that it does not 

 diy up during the process of arrangement. 



When diatoms are mounted on the cover, it is 

 necessary to temporarily attach it to a slide : this is 

 sometimes done with a little balsam. My own plan 

 is to place a minute drop of water on the slide and 

 drop the cover upon it : it will adhere firmly enough 

 to allow of the necessary manipulations. Captain 

 Haig's method is more elaborate. He first centres 

 very carefully an ordinary slide, and then makes a 

 ring the size of the cover he intends using with gold 

 size ; in the centre of this he makes another minute 

 ring. He now heats the shde until the rings burn 

 black ; on the outer ring he places three little pieces 

 of bees-wax ; on these he fixes the cover by slight 

 pressure. When the arrangement is completed, the 

 slide or cover must be placed on the hot plate to 



evaporate the glycerine. 



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