12 ELEMENTARY CHEMICAL MICROSCOPY 



object formed by the objective; but in addition to this the usual 

 type of ocular employed serves as a collector of light rays and 

 increases the brilliancy of the image and therefore of the useful 

 area of the field of view. 



Eyepieces are of two types, those in which the real image is 

 formed inside the lens system of the ocular, and those in which 

 the real image is formed outside the ocular. The former are 

 known as negative or Huygenian eyepieces; the latter, as positive 

 or Ramsden eyepieces. 



Oculars are designated either by their equivalent focal length, 

 by the number of times they magnify the real image formed by 

 the objective or by arbitrary numbers or letters based upon 

 either equivalent focus or magnification. The shorter the equiv- 

 alent focal length the higher the magnification. When desig- 

 nated by their magnification the figures with which they are 

 marked indicate the number of times the real image is magnified. 



The negative or Huygenian ocular is almost universally 

 employed in microscopic work. It consists of two plano-convex 

 lenses mounted convex sides down. Through this construction 

 the lower or field lens becomes optically a part of the objective 

 system since it collects the light rays and reduces the size of 

 the real image formed by the objective. This leads to the pro- 

 duction of a brighter image as seen in the microscope, increases 

 its clearness and because of the reduction in size of the real 

 image the field of the microscope is enlarged. It will be seen 

 on consulting the diagram, Fig. 2, that the light rays cross just 

 above the field lens, this yields, to a considerable degree, a cor- 

 rection for chromatic aberration without the use of combinations 

 of flint and crown glass. 



The lenses in the negative ocular are usually so placed in their 

 mounting that their distance apart is about half the sum of their 

 focal lengths. Theory calls for a focal length of the field lens 

 to be about three times that of the eye lens. In practice this 

 combination rarely obtains. 



The positive or Ramsden ocular consists of two plano-convex- 

 lenses with their convex surfaces turned toward each other (see 

 ocular shown in Fig. 27) and the entire combination acts as a 



