18(> SELECTION AND USE 



them with the same water, mud, etc., in which they were origi- 

 nally found. It will sometimes, however, be- well to filter the 

 water so as to remove all such inhabitants as are apt to eat up 

 the others. Water, may be filtered through paper, or where fil- 

 tering paper is inaccessible, the neck of a funnel may be loosely 

 plugged with cotton. Even this does not quite free it from 

 noxious eggs or germs, and we have sometimes boiled it in the 

 flask in which the objects were to be kept. The mouth of the 

 flask was then plugged with loose cotton, and when the water 

 was cold, the objects we wished to preserve were introduced. 



"When floating freely in these diminutive aquaria, many ob- 

 jects are as difficult to find and capture as would be a small fish 

 in a large pond. The microscopist, therefore, requires special 

 means for capturing them, and placing them on a slide. For 

 this purpose nothing serves so well as what are known as dip- 

 ping or fishing tubes. These are simply glass tubes of different 

 diameters (from one tenth to. one quarter of an inch), and of 

 any convenient length. They are used by closing the upper 

 end with the finger, bringing the lower end near the object (un- 

 der the water), and then removing the fingers from the uppei 

 end. The water, in seeking to find its own level in the tube 

 rushes in with great force and carries the object with it. By 

 again placing the finger on the upper end of the tube, the latter 

 may be lifted from the bottle, and the water with it, and by a little 

 dexterous management it is easy to cause the object to flow out 

 on a slide without allowing too much water to go with it. 

 These tubes are made straight, curved, and with one end drawn 

 to a point, but for most purposes the straight tube answers best 

 as it is most easily kept clean. We prefer to grind the ends 

 rather than to make them smooth by fusion, as the latter pro- 

 cess generally contracts the opening, and renders the tube dif- 

 ficult to clean. The best plan, however, is to heat the upper 

 end strongly before the blowpipe, and turn the edge outward 

 like the mouth of a test-tube. It is then easily closed, and the 

 tube is very strong. The lower end should be ground. 



