132 UNSTAINED PREPARATIONS 



1. Examination of a culture on an ordinary slide. 



A. Cultures in fluid media. 1. Prepare an absolutely clean slide and 

 cover-glass. 



2. Aspirate a few drops of the culture into a Pasteur pipette, taking care 

 of course not to introduce contaminations. 



3. Pick up a cover-glass by one of its corners with a pair of Cornet's forceps, 

 and let fall a drop of the liquid from the pipette on to the centre of the cover- 

 glass. 



4. Invert the cover-glass on to a slide and the drop will spread out in a 

 thin layer. One must be careful not to introduce any air bubbles as these 

 would interfere with the subsequent examination. 



5. Place the preparation on the stage of the microscope and examine with 

 a No. 8 or No. 9 objective and a No. I. or No. II. eyepiece. If the examination 

 is likely to be prolonged the edges of the cover-glass can be luted with 

 paraffin in the following manner : Soak up the excess of culture fluid which 

 has exuded from the edges of the cover-glass with a cigarette paper or piece 

 of filter paper : then apply a heated iron rod it is better to use a special 

 instrument such as that shown in fig. 120 to a block of paraffin, so as to 



FIG. 120. Instrument for luting with paraffin. 



melt a little of it : in doing this some of the paraffin will adhere to the rod 

 and can be transferred to each of the corners of the cover-glass to fix it in 

 position. Then by taking up some more paraffin on the rod the edges can 

 be luted. 



The pipette with which the culture was removed should not be used again. 

 Pipettes which have been in contact with a culture must never on any account 

 be laid on the bench. All pipettes after use should be put into a metal vessel, 

 and when the experiment in hand is completed sterilized either in the autoclave 

 or more readily by boiling for a few minutes : only then can they be safely thrown 

 away. 



B. Cultures on solid media. 1. Take a cover-glass in a pair of forceps, and 

 put a little drop of recently filtered water (Chamberland filter) or sterile 

 broth in its centre. 



2. Open the culture-tube in the ordinary way, take up a trace of the culture 

 on a platinum wire and re-plug the tube. 



3. Make an emulsion of the culture in the drop of water on the cover-glass 

 with the wire. Flame the wire. 



4 and 5. As above. 



A common mistake is to remove too much of the culture. If more than a trace 

 be taken, there will be too many organisms in the field of the microscope and' the 

 examination of them will be exceedingly difficult. It cannot be too clearly under- 

 stood that the fewer the organisms the better can their shape, movements, etc., 

 be studied. 



2. Hanging drop preparations. 



By using a hollow-ground slide any organism under examination can be 

 kept alive for a long time and its development studied. 



(i) The technique of the hollow-ground slide. 



There are many patterns of slides or cells for use with the microscope. 

 A. Koch's hollow-ground slide. This is simply a slide of the ordinary size 



