206 MICROSCOPIC OBJECTS. 



size, and which by the action of the joint and the pivot 

 which fastens it down allows a double motion to the 

 mirror. Two plates of brass curved to the requisite radius 

 are firmly fixed by 8 screws in each to the table ; their 

 arc is 90*=. h is a square arched piece of brass of the 

 proper curve, suited to its distance from the pivot of the 

 hinge, which passes into a brass box stuffed with cork, 

 and on the outside of which is turned a circular groove, 

 into which the edges of the two pieces of brass are fitted, 

 so that the box shall traverse between them as the mirror 

 is turned round on its pivot, but at the same time not 

 too easily, which is effected by making the two plates 

 spring a httle in opposite directions. At fig. 5 (p. 202) is 

 a section of this apparatus of half the real size. A 

 pinching- screw and spring may be substituted for the 

 piece of cork, if preferred ; and an ordinary mirror of 

 double action may be used, instead of the form here 

 described, by those who like it better ; but it will not 

 act so well unless of unseemly dimensions in many cases, 

 especially in throwing light downwards when the object- 

 glasses are posited underneath the stage, or in affording 

 that beautiful lateral or black ground illumination re- 

 commended by Rev. Mr. Reade (in his Essay pubhshed in 

 the * Micrographia,') and which certainly brings out the 

 colours of transparent objects better than any other. 

 To effect this with the present apparatus, it is merely 

 requisite to turn the stage round a little on its pin (A), 

 and to fix the body at right angles to its plane. In this 

 position, then, the natural action of the mirror will give 



