DIRECTIONS FOR USING THE MICROSCOPE. 775 



white by plaster of Paris. This is mounted on brass, and fitted over 

 the frame of the ordinary silvered mirror, thus not requiring the 

 latter to be removed. The advantage is, that the whole rays reflected 

 from the surface of plaster of Paris are brought into one focus, together 

 with those reflected from the surface of the glass, and thus an equal 

 and brilliant light is. produced. In viewing opaque objects, the light 

 is thrown by the condenser directly on the object, and sometimes a 

 metallic speculum, called a Lieberkuhn, is connected with the object- 

 glass, by means of which an additional supply of light is obtained. 

 In conducting microscopic observations great steadiness of the in- 

 strument is required, which should accordingly be set upon a very 

 firm and sufficiently large table, so that all the apparatus hitherto 

 mentioned shall be within reach of the observer. It is proper also to 

 begin the examination of objects with the lower magnifying powers, 

 and to pass gradually from them to the use of the higher powers. 

 By such means a far larger portion of the object is seen, and a more 

 correct idea is obtained of the relations of the parts when considered 

 as a whole. Object-glasses, varying from 30 to 50 diameters, are the 

 best to begin with. The eye-glass of lowest power, that is, the longest 

 one of the series, is also the one which ought generally to be used 

 in the first instance, and as Jlong as the power can be increased by 

 object-glasses of greater magnifying power, any more powerful eye- 

 piece should not be used, for it must be remembered that the eye- 

 piece merely magnifies an image produced by the object-glass. If, 

 therefore, there be any defect in the image, it is magnified by the 

 eye-piece. 



Directions by Smith and Beck for using the Compound Microscope. 



Before using the microscope, see that the mirror, object-glass, and 

 eye-piece, are free from dust : — a little soft wash-leather should be 

 used in cleaning these. The instrument should be placed on a steady 

 table to avoid vibration. The best position for examination by day- 

 light is with the window to the left hand, and the back partly turned 

 toward the window, so that the light may fall directly upon the 

 mirror, and not upon the observer's face. At night, when a lamp is 

 used a shade should be placed if possible before the lamp, so as to 

 screen the eyes from its glare. The nearer the observer can approach 

 the window by day, and the closer the lamp can be brought towards 

 the mirror at night (say from fifteen to twenty inches), the better ; as 

 all the light that can be obtained is required for high magnifying 

 powers ; and if too intense for some objects, can be easily modified by 

 the mirror. When the microscope has a joint to the stand, it should 

 generally be used with the body in an inclined position — at an angle 

 of about 45°, this being much more convenient for the observer, and 



