several changes with consequent results can be noted : 



If the objective is of shorter focal length, a larger 

 real image is formed at c d. 



If the distance between objective and eyepiece is 

 increased, a larger real image is formed at c d. 



If the eyepiece is of higher power, a larger virtual 

 image is formed at e f. 



In the same manner a reduced magnifying power 

 may be obtained by reversing these conditions. 



As has already been stated, a i inch lens with a 

 distance of ten inches between it and the image gives 

 a power of ten diameters, and the eyepiece multiplies 

 the virtual image by the extent of its power. From 

 this it can be easily computed that with a i inch 

 objective used with a tube length of ten inches and 

 i inch eyepiece, a magnifying power of lox 10 = 100 

 will be obtained ; or the same combination with a tube 

 length of five inches will give one-half this power or 

 fifty. 



Objectives are divided into two classes, dry and 

 immersion. In the dry objectives there is no interven- 

 ing medium other than air between the lowest lens 

 surface of the objective and the upper surface of the 

 cover glass. In the immersion objective a liquid fills 

 this space. From this fact it is easily seen that liquid 

 can only be used with objectives which are quite close to 

 the cover and therefore short focus or high power, and 

 so, on the other hand, objectives of long focus or low 



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