10 THE STRUCTURE OP 



ture, but in the better class of instruments it is 

 simply attached to 

 the under part of 

 the stage by a bayo- 

 net catch, or by a 

 sliding plate of me- 

 tal (fig. 5), and can 

 be readily removed p^, 5. Diaphragm, 



therefrom when it 



is desirable to employ other methods of illumi- 

 nation. 



In working with the Microscope it is necessary 

 to adopt some artificial means for ensuring a larger 

 supply of light than can be obtained from the 

 natural diffused light of day, or from a lamp or 

 candle. For this purpose the Microscope is fur- 

 nished with a double mirror, ff, having two reflect- 

 ing surfaces, the one plane and the other convex. 

 The latter is the one usually employed in the illu- 

 mination of transparent objects ; the rays of light 

 which are reflected from its concave surface are 

 made to converge, and thus pass through the object 

 in a condensed form to the eye. The plane mirror 

 is used generally in conjunction with an achromatic 

 condenser, when parallel rays only are required. 

 The whole apparatus is attached to that portion of 

 the hollow pillar continued beneath the stage, in 

 such a manner that it can be moved freely up and 

 down the stem that supports it. This motion 

 enables the Microscopist to regulate the intensity 

 of his light by increasing or decreasing the distance 

 between the mirror and the stage ; while the 

 peculiar way in which the mirror itself is suspended 

 on two points of a crescent-shaped arm, turning on 

 a pivot, gives an almost universal motion to the 

 reflecting surfaces. The observer by this means 

 can secure any degree of oblique illumination he 



