INTRODUCTORY CHAPTER. 9 



which is usually made of glass. If one side of a convex lens is presented to the 

 parallel rays of the sun, they are made to converge on the other side to a single 

 point, called the focus, as is easily seen by receiving them upon a piece of paper, 

 placed at a proper distance from the centre of the lens. This distance is called 

 the focal distance of the lens. On the other hand, if an object is placed in this 

 focus, the rays of light, which proceed from it and fall diverging upon the nearest 

 side of the lens, emerge parallel on the other side, and if they are then received 

 by the eye the object is clearly seen. Now it is found that the apparent length 

 and breadth of an object is magnified by such an arrangement, in the ratio of the 

 limit of distinct vision to the focal distance of the lens. Thus, if the limit of dis- 

 tinct vision is Jive inches, and the focal distance of a lens through which the 

 object is viewed one inch, the object will be seen Jive times nearer than by the 

 naked eye, and will be magnified Jive times both in length and breadth ; and its 

 surface will be increased in apparent size twenty-Jive times. Single lenses of 

 garnet have been executed of very small focal distances. Two of this kind are 

 in the possession of Sir David Brewster, the focal lengths of which are between 

 one-thirtieth and one-fiftieth of an inch. A Figure i. 



lens with a focal distance of one-fiftieth of an 

 inch would magnify an object linearly two 

 hundred and fifty times, and superficially, 

 sixty-two thousand five hundred times. Such 

 a lens as has been described is termed a Sin- 

 gle Microscope, and is represented by Figure 1, 

 where A B is the lens, C D an object placed 

 in the principal focus of the lens, at the dis- 

 tance H I, and E F the magnified image seen 

 at the distance of distinct vision by the eye 

 at N. The image exceeds the object in 

 length and breadth as much as N K is larger 

 than H I. 



COMPOUND MICROSCOPE. However valuable the Single Microscope may be, 

 to the scientific observer in many cases, it is not possessed of sufficient power 

 to reveal, in their perfect developments, the more minute objects of his investi- 

 gations, and for this purpose the Compound Microscope is employed. The two 

 essential parts of this instrument are an object-glass and an eye-glass. The office 

 of the object-glass is to produce a magnified image of the object, which ima^e 

 is again magnified by viewing it with the eye-glass, as if it was an object : the 

 eye-glass being in fact a single Microscope. An image is formed by the object- 

 glass in the following manner : 



The object to be magnified is placed a little beyond the focus of parallel rays, 

 for if the object were placed exactly in this focus, the rays proceeding from any 

 point on its surface would emerge parallel to each other on the opposite side and 

 never meet ; but by placing the object a little beyond the focus, the rays con- 



