FIELD-LENS. 



image o' o', at a less distance from the object-glass L L. The 

 dimensions of this image are so small, that every part of it can be 

 seen at once with the eye-glass. 



The portion of the image which can be seen at once with the 

 eye-glass, is called the FIELD OF YIEW of the microscope. 



It is evident from what has been stated, that the effect of the 



Fig. 2. 



lens F F is to increase the field of view, since by its means the 

 entire image of the object can be seen, while without its inter- 

 position the central parts only would be visible. 



The lens F F has, from this circumstance, been called the 



FIELD-LENS. 



But the increase of the field is not the only effect of this 

 arrangement. 



The light which would have been diffused over the surface of 

 the larger image o o, is now collected upon that of the smaller 

 image o' o' ; and the brightness, therefore, will be increased in the 

 same proportion as the surface of o o is greater than the surface of 

 o' o', that is, in the proportion of the square of o o to the square 

 of o' o 7 . 



Another effect of the field-lens is to diminish the length of the 

 microscope, for the eye-glass, instead of being placed at its focal 

 distance from o o, is now placed at the same distance from o' o'. 



7. In this brief exposition of the general principle of the micro- 

 scope, the image, which is the immediate subject of observation, 

 is supposed to be produced by a convex lens ; such an image, 

 however, may also be produced by a concave reflector, and being 

 so produced may be viewed with an eye-glass, exactly in the same 

 manner as when produced by a convex lens. 



Microscopes have accordingly been constructed upon this 

 principle, and are distinguished as REFLECTING MICROSCOPES ; 

 those in which the image is produced by a lens being called 



REFRACTING MICROSCOPES. 



The principle of a reflecting microscope will be understood by 

 reference to fig. 3, where L L is the concave reflector, of which c 

 is the centre ; the object o is placed towards the reflector, at a, 



5 



