THE MICROSCOPE. 



lens (L). To him the rays from A would appear as if they 

 proceeded from a. He would therefore see an enlarged 

 virtual image (a b) of the object. This is the principle of 

 the simple microscope. Fig. 19 explains the theory of en- 

 largement by the simple microscope more clearly. Let A 



a 



FIG. 19. The simple microscope. 



B be an object placed within but near the principal focus 

 of the lens L, the rays from A B are rendered less divergent 

 by the lens, and are focalised by the crystalline lens of the 

 eye upon the retina. To the observer they appear as if 

 they proceeded from an object in the position a b at the 

 distance of ten inches from the eye. 



The simple microscope is often used for dissection ( 50) 

 and other purposes, where a high magnifying power is not 

 required. 



28. Compound Microscope. The compound micro- 

 scope is always employed where high magnifying powers 

 are needed. Its simplest form (Fig. 20) consists of two 

 convex lenses, one the objective (O) placed near the 

 object, the other the ocular (O'), placed next the eye. 

 The object (a b) is placed slightly beyond the focal length 

 of the objective O. A real magnified image is produced 

 at a' b' c, slightly within the focal distance of the ocular O', 

 by which it is again magnified, so that the observer sees 

 the virtual magnified image ABC. The compound 

 microscope is therefore really just a conjunction of two 



