THE MICROSCOPE. 



where the object' is placed. The positive and negative ends of 

 the conductor of the electric machine, or the poles of a voltaic 

 battery, being put in connection with the handles b b of the 



Fig. 23. 



platinum wire, the spark or current will pass from the point of 

 one of the wires a a to that of the other, being transmitted through 

 the object placed between them. 



TIIE ILLUMINATION OF OBJECTS. 



43. Among the accessories of .the microscope, there is none the 

 right use of which is more important than the illuminators. By 

 the proper application of these, an infinite variety of beautiful 

 effects are produced, and an infinite number of interesting con- 

 sequences developed, while by their abuse, and by the misconcep- 

 tion and misinterpretation of their indications, the most fatal 

 errors and illusions may arise. 



Let any one, however inexperienced in the manipulations of a 

 microscope, applying one hand to the mirror and the other to the 

 disc of diaphragms, vary at pleasure the position of the former, 

 and turn the latter slowly round its centre, thus shifting the 

 direction, and varying the quantity of the light which falls upon 

 the object, and he will witness, in looking at the object through 

 the instrument, a series of appearances which will soon demon- 

 strate to him how curious, complicated, and important a part the 

 illuminators play in microscopical phenomena. 



44. Objects may be rendered visible in two ways, either by 

 light reflected from those parts of their surfaces which are pre- 

 sented towards the observer, or by light falling on the posterior 

 surface, and partially transmitted through them. Opaque bodies 

 can be seen only in the former way, but translucent objects may 

 be seen in either of these ways. 



A translucent object presents a different appearance, according 

 us it is seen by a front or back light. The leaf of a tree or plant, 

 seen by reflected light, appears to have some particular tint of 

 42 



