780 MICEOSCOPIO OBJECTS. 



is the instrument to any smaJl alteration of focus. The focal adjust- 

 ment is made and varied by what is called a fine adjustment screw ; 

 and the accurate adjustment of the object is judged of by the sharp- 

 ness of the delineation of the image, as well as by the fineness and 

 cleaxness of the outline. An experienced microscopic observer always 

 uses the instrument with his finger and thumb grasping the fine 

 adjustment screw, and would not be content . with his observation, 

 although it was limited to a mere peep of the object, unless he had 

 made the fine focal adjustment for himself. 



Prepaeation and Selection of Objects foe Examination. 

 — Opaque objects require merely to be made smooth or level on one 

 side, and to be fixed on the other. If the object is to be viewed by 

 transmitted light, a section or slice suflSciently thin must be procured, 

 a common sharp scalpel or razor are the instruments to use. The object 

 must be moistened with water, and sometimes it is advisable to make 

 the section under water. If the object is very small it may be em- 

 bedded in solid paraflSn (an ordinary parafiBn candle), by melting it 

 and pouring it over the object, and allowing it to cool ; or it may be 

 embedded in gum arable in the following manner : — Make a cone of 

 blotting paper about the size of the end of the finger, half fill it with 

 a solution of gum as thick as possible, place the piece of tissue in the 

 gum, and then set the cone in a vessel containing three or four times 

 its bulk of rectified spirit for an hour or so, in order that the spirit 

 may remove the water from the gum. Lastly, expose the cone to 

 the air in a warm place, until the gum is hard enough to be cut. In 

 making the sections wet the knife with water, and lay the sections 

 in water to remove the adherent gum. A small piece of tissue may 

 also be supported for the purpose of section between two slices of 

 carrot or cork. Sections should be made in various directions, so that 

 a correct knowledge may be obtained of the relation of the component 

 parts. Maceration in water, and tearing the parts asunder with fine 

 needles, are the best methods for obtaining the ultimate tissues of 

 plants. Thin glass plates to cover the object under the microscope 

 must be invariably used. They keep the object moist, they prevent 

 the object-glass from being covered with vapour, and so rendered 

 obscure; and, lastly, they produce a slight pressure, by which the 

 elementary parts of the substance may become separated from each 

 other, so as to lie on one plane. The thin covers are not absolutely 

 necessary where very low powers are used. In placing the object 

 on the stage care must be taken not to bring it in contact with 

 the object-glass of the instrument. It is also to be remembered 

 that, in a compound microscope, the image is inverted, and that, 

 consequently, the object is moved in a direction contrary to that of 

 the image. 



The following list of tissues to be examined by the student of 



