2 THEORY OF THE MICROSCOPE. 



Pi* the distances of the images b' a and b" a" from the eye-piece, 

 and by /j the focal length of the latter, we obtain the equation 



JL l 1 



Pi l>* A ' 



If the distance between the eye-piece and the objective is con- 

 stant, and their focal lengths / and / x are given, then p.* is deter- 



B 



mined by p^ j^ by j>*, and />* by />, and, consequently, the first 

 ([uantity 2^1* by the last p. It is always possible, therefore, so to 

 regulate the distance of the object from the objective that the final 

 virtual image shall be at the distance from the eye of distinct vision. 

 The linear dimensions of the image and object are in the ratio of 

 their respective distances from the image-forming lens. In the 



p* 

 case of the objective this relation is expressed by 



and in that 



