40 



THE ART OF MAKING MICROSCOPE SLIDES 



Diatoms 



that the film be applied by dipping a 

 needle into the cement and then drawing 

 the flat of the needle sharply across the 

 covershp. This leaves an invisible film of 

 dry gelatin on the cover, and individual 

 diatoms may be placed on this film to 

 which they will not adhere until the film is 

 slightly moistened by breathing on it. The 

 moment this has been done the diatoms 

 are permanently attached. 



Before individual diatoms may be se- 

 lected for this technique, however, they 

 must be dried. It is not safe to dry them 

 on glass, to which they frequently adhere. 

 It is better to attach a piece of mica to a 

 slide with petrolatum and to evaporate the 

 drop of water on this base. Individual di- 

 atoms may then be picked up on the end 

 of a hair under the microscope. There is 

 usually enough grease on a normal hair to 

 permit the diatom to adhere. Bellido de- 

 scribes the ingenious idea of mounting a 

 hair on the collar of a microscope objective 

 in such a manner that the tip of the hair 

 is in focus when the draw tube of the 

 microscope is pulled halfway out. It fol- 

 lows that when the draw tube is pushed 

 home the hair will be out of focus, and also 

 well above the plane of the object which is 

 in focus. It is possible, therefore, to press 

 the draw tube fuU}^ home, search one of 

 the squares of mica for the required speci- 

 men, pull out the draw tube until the hair 

 is in focus, and then lower the microscope 

 until the hair touches and picks up the 

 object. Belhdo recommends that the hair 

 be moistened with a little bromonaph- 

 thalene and that the film of gelatin also be 

 lubricated with the same reagent. He has 

 also described, in the place quoted, a 

 sealed chamber within which all these 

 operations may be conducted without the 

 risk of dust faUing on the preparation. 

 Another device for handling individual 

 diatoms on a mechanically operated hair 

 has been described by Meakin 1939 

 {Microscope, 4:8). These mechanical de- 

 vices are only necessary, however, for 

 handling large quantities of rather small 

 diatoms. A few months of practice with a 

 hair mounted in any holder will enable the 

 average worker to arrange diatoms di- 

 rectly. A mechanical device is, of course, 

 almost necessary if one is endeavoring to 



arrange the diatoms according to any 

 artistic pattern. In any event, as soon as 

 the diatoms have been arranged one 

 breathes very gently on the coated cover- 

 slip and pauses a moment or two. The 

 covershp is then examined under the 

 microscope and a few of the larger and 

 rougher diatoms very delicately probed 

 with a hair. If they are found to be firmly 

 attached, it may be assumed that the 

 smaller diatoms are also attached. If, how- 

 ever, anything is found to be loose, one 

 breathes again, and again probes until 

 such a time as all the diatoms are fixed. 

 These coverslips with the diatoms at- 

 tached to them may now be laid on one 

 side while the necessary cells for mounting 

 are prepared. 



There will be required a turntable, 

 shdes, a fine brush, a hot plate, and some 

 de-waxed shellac (Chapter 28 V 11.2 

 Hitchcock 1884) which should be as thick 

 as can conveniently be persuaded to flow 

 from a brush of the size selected. A fine 

 ring is then turned of a size shghtly 

 smaller than the covershp. Save for the 

 very largest diatoms a single ring of this 

 cement will be sufficiently thick. It is ab- 

 solutely necessary that these shellacked 

 rings be baked if they are to become in- 

 soluble in the bromonaphthalene used for 

 mounting. As soon, therefore, as the alco- 

 hol has evaporated from the shellac, the 

 slides are placed on a hot plate, or for that 

 matter, in an oven, and heated to just 

 below the melting point of the shellac for 

 at least 30 minutes. The cells, which 

 would now be adequate for aqueous media, 

 must be further processed for bromo- 

 naphthalene mounting by having the top 

 ground flat. This is done by taking some of 

 the finest available carborundum, making 

 it into a slurry with water, and spreading 

 this on a sheet of fine quality plate glass. 

 The slide, with the cell down, is then laid 

 on this glass and, with a delicate finger 

 placed over the center of the cell, moved 

 gently backward and forward for a few 

 moments. It is then picked up, washed 

 under the tap, and examined with a strong 

 lens to make sure that there is a flat, 

 smooth area over the whole of the top of 

 the cell. The slide is then washed free of 



