PART I. 

 THEOBY OF THE MICEOSCOPE. 



I. 



INTRODUCTION. 



THE compound dioptric Microscope consists, in principle, of two 

 systems of convergent refracting lenses, of which the one directed 

 to the object is termed the Objective, and the other, turned to the 

 eye, the Eye-piece. The objective forms an inverted image of an 

 object placed somewhat beyond its principal focus, and this image is 

 viewed through the eye-piece just as through an ordinary magnify- 

 ing lens. 



The optical action of the Microscope may be readily compre- 

 hended from Fig. 1, which shows the process of the formation of 

 the; image. The rays of light proceeding from the object a b unite 

 after their passage through the objective A to form the real image 

 V a, the position and magnitude of which depend solely upon the 

 focal length of the objective and the distance of the object. If we 

 denote these quantities by/ and p respectively, and the distance of 

 the image by j?*, then their mutual dependence is expressed by the 

 well-known equation 



.1 _! I 



P P* ~ 7 ' 



The image ~b' a acts as a real object to the eye-piece B, and as its 

 distance is regulated so that it lies somewhat nearer to the eye-piece 

 than its principal focus, a virtual image V a" is formed, which is 

 visible to the eye when placed over B. If we denote by p l and 



B 



