THE COMPOUND MICROSCOPE 



direct rays of light reach the objective, such as do 

 pass into the microscope are all diffused from the 

 edges of the object. 



We have already mentioned that the optical parts 

 of the compound microscope are of greater import- 

 ance than what may be termed the mechanical 

 portions and the objectives are more important than 

 the eyepieces. Better results can always be obtained 

 with a good, high-power objective and a low-powe>" 

 eyepiece, than with an inferior objective and a good 

 quality eyepiece. The merits of the eyepiece, how- 

 ever great, will not be adequate compensation for 

 the failings of the objective. Modern objectives are 

 composed of several lenses and of a combination of 

 flint and crown glass, as we explained in our last 

 chapter. They are so designed that they can be 

 screwed into the lower part of the microscope tube. 

 The focal length of each objective is, or should be, 

 marked upon it ; as a general rule, however, it may 

 be taken that the smaller the lower lens, the shorter 

 its focal length and therefore the greater its magni- 

 fying power. 



The form of eyepiece most usually met with is 

 known as Huygen's. It consists of two plano-convex 

 lenses, with their flat or plane surfaces directed 

 away from the objective. The smaller of the two 

 lenses is situated nearer to the eye of the observer 

 and is known as the eyeglass; its function is to 

 magnify the image formed by the objective. The 

 larger, lower lens is known as the ffeld or collecting 

 glass; it renders the image clearer though, in so 



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