INTRODUCTORY CHAPTER. 11 



the limit of distinct vision exceeds the principal focal distance of the lens G H. 

 Thus, if the object A B, is within half an inch of the centre of the lens, and the 

 image F E is formed at the distance of twelve inches, the linear magnifying 

 power of the object-glass is twenty-four; and the superficial magnifying power 

 24x24, or five hundred and seventy-six. If now, this image is distant from 

 the centre of the eye-glass G H, one quarter of an inch, and the limit of distinct 

 vision is five inches, the linear magnifying power of the eye-glass is twenty, and 

 the superficial magnifying power 20 X 20, or four hundred. The entire linear mag- 

 nifying power is, therefore, expressed by 24X20, equalling 480, and the superfi- 

 cial magnifying power by 576X400, or 230,400. The image F E is, therefore, 

 twenty-four times longer and broader than the object A B, and its surface five 

 hundred and seventy-six times greater while the length and breadth of the 

 image K L, are twenty times greater than the like dimensions of F E, and its sur- 

 face four hundred times greater. Comparing then the second image K L, with 

 the object A B, the length and breadth of the former are 480 times, (24X20,) 

 greater than the length and breadth of the latter, and the surface of A B is contained 

 in that of K L two hundred and thirty thousand four hundred times, (576 X 400.) 



When a minute object is thus highly magnified the whole of its surface can- 

 not be seen at once, but only a small portion of it, and that extent of surface 

 which is visible at one time is called the field of view. Such is the Compound 

 Microscope in its most simple form, and thus constructed it always possesses 

 many serious and fatal defects. These have only been removed by the aid of 

 complicated arrangements, based upon the most refined scientific principles, and 

 executed with consummate skill. 



The Compound Microscope has thus at length attained a very high degree of 

 perfection, and for the sake of instruction we shall describe the Microscope of M. 

 Chevalier, a French optician of distinguished ability. In Figure 4, an outline 

 is presented of the parts Figure 4. 



which are employed in 

 magnifying the object, and 

 in Figure 5, (frontispiece,) 

 the complete instrument is 

 delineated. In Figure 4, 

 B R A is the object, and 

 O P the object-glass. The 

 rays of light, proceeding 

 through this lens from the 

 object, fall upon glass 

 prism C D F, in such a 

 manner that they are to- 

 tally reflected from the in- 

 clined surface C E F, and 

 emerging from the side 

 C D, fall upon the lens 

 G V H ; the two central 



