OF THE MICBOSCOPE. 193 



pan is white, chalk or sulphate of baryta being substituted for 

 lampblack. The pan with a transparent bottom is of precisely 

 the same size, except that the depth is but half as much the 

 extra depth in the other pan being filled with wax. A quarter 

 of an inch is a sufficient depth of liquid for most objects, and 

 when the sides of the pan are higher than necessary they inter- 

 fere with the use of knives and needles. 



Dissections may also be carried on in watch-glasses, though they 

 are not quite as convenient as pans with perfectly flat bottoms. 

 The kind known as lunette glasses should be chosen, as they 

 iire flat in the centre. When a watch-glass is used for this pur- 

 pose, it is necessary to cement it into a hole cut in a thin piece 

 of wood about four inches long, and of a width which is rather 

 greater than the diameter of the glass. 



Most of this work is, of course, done under a simple micro- 

 scope. The Excelsior, when screwed to a larger base, as de- 

 scribed on page 42, answers very well. Larger and more ex- 

 pensive dissecting microscopes are supplied by most opticians. 



In addition to these general methods, which are applicable 

 to a great variety of subjects, there are a few special processes 

 which must be adopted in particular cases. In some instances, 

 as when the line of investigation is a new one, the microscop- 

 ist must work out his own processes, but the following special 

 cases will probably prove interesting to beginners. 



It frequently happens that the objects for which the micro- 

 scopist is searching are found mixed with coarser materials, 

 and in this case it will be found possible to effect a separation 

 by the process known as elutriation or washing. Mix the mat- 

 ter thoroughly with water in a tall jar and allow it to settle. 

 In a short time say one minute the very coarse particles will 

 have fallen to the bottom, and if the liquid be now poured off 

 and allowed to settle, the finer portion will be found in the 

 second vessel. By graduating the time and carrying the pro- 

 cess out to its full extent, a wonderfully perfect separation may 

 be effected. Diatomaceous earth may frequently be treated in 

 this way to advantage. 



In some cases separation must be effected by burning, or the 



action of chemical agents. Guano and various organic matters 



interesting residues after everything soluble has been 



