234 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



I. THE MAGNIFYING GLASS, OR SIMPLE MICROSCOPE. 



A simple convergent, piano- or bi-convex lens, mounted in a form 

 which varies according to its use, is a microscope reduced to the 

 greatest simplicity. This is usually called a magnifying glass or 

 simple microscope. 



Fig. 170 represents the path of the luminous rays in the magni- 

 fying glass. The object AB is placed at a point nearer the lens than the 

 principal focus. The eye, placed at the converging point F, receives 

 these rays as though they were sent from the points A'B', that is to 

 say, a direct virtual magnified image of the object. 



FIG. 170. Path of the luminous rays in the small microscopes. 



In order that this image be sharp, it is necessary that the distance 

 A'F be equal to that of distinct vision, from which it follows that 

 the object must be placed at a fixed point, found by calculation or 

 more easily by actual trial. Very near the principal focus F, and the 

 greater the curvature of the lens, that is, the shorter its focus is, the 

 nearer to this point the* object must be. If the object is placed fur- 

 ther from the lens, it soon reaches the principal focus /, and the image 

 diminishes in size. If, on the other hand, the object is brought nearer 

 to the magnifying glass, the size of the image increases, but it becomes 

 ill-defined. 



Magnifying power of the lens. In. optical instruments the magni- 

 fying power in the case of distinct vision is nothing more than the 

 ratio between the apparent diameter of the object, and the apparent 

 diameter of the image. By this is understood the value of the angles 



