OBJECTIVES AND OCULARS II 



Never use a high power until the preparation has first been 

 examined and centered with a low one. Remember that it is 

 possible to see more of the object and see it better with low 

 powers than with high ones. 



Invariably work with the lowest power which will clearly 

 define the preparation. The most common fault of the beginner 

 is to employ too high a magnification. 



Oculars. The function of the ocular or eyepiece of a com- 

 pound microscope is to magnify the real inverted image of the 

 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 

 equivalent focal length the higher the magnification. When 

 designated by their magnification the figures with which they 

 are marked indicate the number of times the real image is mag- 

 nified. 



In all ordinary microscopic work negative or Huygens oculars 

 are employed, the use of positive or Ramsden oculars being 

 restricted to micrometer eyepieces. In the case of positive 

 oculars the entire lens system acts as a magnifier. 



The usual type of negative eyepiece is shown in section in Fig. 2, 

 with the passage of the light rays diagrammatically indicated. 

 It will be seen on consulting the diagram that the lower or field 

 lens, as already stated, collects the light rays and reduces the 

 size of the image formed by the objective and is thus, optically, 

 in reality a part of the objective system; the eye lens functions 

 as a magnifier of the image formed by the field lens. It is evident 



