190 Experimental Philosophy. [Lecture 13. 



inches ; and many objects are too small to be 

 seen at that distance. This deficiency is supplied 

 by the microscope. 



The single microscope is only a small convex 

 glass cd, (fig. 70,) having the object ab placed in 

 its focus, and the eye at the same distance on 

 the other side ; so that the rays of each pencil, 

 flowing from every point of the object on the side 

 next the glass, may go on parallel in the space 

 between the eye and the glass; and then, by 

 entering the eye at C, they will be converged to 

 as many different points on the retina, and form 

 a large inverted picture AB upon it, as in the 

 figure. 



If, as in fig. 71, which represents the effect of 

 this microscope, the object AB is in the focus of 

 the lens DE, and the eye is in the other focus F, 

 as much of the object will be visible as is equal 

 to the diameter of the lens ; for the rays AD and 

 BE proceed through the extremities of the lens, 

 and are united at F. Hence a maxim in optics 

 that when an object is placed in one focus of a 

 lens., and the eye in the other ^ any lineal dimen- 

 sion of the object appears just twice as large as it 

 would to the naked eye, whatever the size of the 

 lens. For the lines FD and FE, if protracted as 

 far as A and B, would form an image exactly 

 twice as large. If the eye is nearer to the lens 

 than the focus, it will see the object still larger; 

 and if it is further off than the focus, it will not 

 see it so large. 



