H'J MICROSCOPIC ILLUSTRATIONS. 



length and distance asunder of their component lenses, and 

 as they usually bear a certain relation to each other when 

 their construction is alike, their power is sufficiently dis- 

 tinguished by simply expressing the focus of the eye-lens. 

 Thus we say this is an inch eye-piece, meaning that the 

 sidereal focal length of the eye-lens is one inch. The 

 acting length of the body is the distance from the object- 

 glass d, to the diaphragm c (fig. 9), though it is usually 

 expressed by the length of the tube (fig. 8.) 



I have been thus explicit, as the magnifying power 

 of an instrument is dependent on the focal length of 

 the objective, the power of the eye-piece, and length 

 of the body, taken together. 



