THE MICROSCOPE. 



11 



PHYSICAL PRINCIPLES. 



The image produced by a compound microscope is a magnified 

 inverted real image. Imagine an innumerable number of lines 

 proceeding from the object to be examined in a direction perpen- 

 dicular to the long axis of the objective. These lines represent the 

 rays of light proceeding from all points of the object. As they pass 

 through the objective they are bent, or refracted, and converged to 

 a common point called the principal focus. No image is produced 

 at this focus. The accompanying diagram illustrates how this focus 

 is produced. 



A B, object; C, lens; F, principal focus. 



It is at the conjugate foci that the image is formed. There are 

 as many conjugate foci as there are emergent points on the sur- 

 face of the object. They are situated between the principal focus 

 and the eye-piece. Let us suppose an innumerable number of emer- 

 gent rays of light to have proceeded from any point of the object. 

 These rays, striking the lens of the objective upon every point 

 of its surface, are refracted and converged to a point called the 

 conjugate focus. The following diagram will illustrate how the 



A B, object; O, objective; A and B', conjugate foci; E, eye-piece; A B', in- 

 verted image produced by the objective; A" B" magnified inverted image viewed 

 by the eye. 



