THE MICROSCOPE. 



tube T is drawn out so as to lengthen the body, and the stage z 

 is brought nearer to the object-glass T. The effect of this is to 

 increase the dimensions of the optical image produced in the eye- 

 piece by the object and field glasses, as explained in 6. 



If a greater increase of magnifying power be desired, the eye- 

 piece may be withdrawn, and a shorter one substituted for it. 



But these expedients are only useful when the increase of power 

 required is confined within comparatively narrow limits. All 

 greater amplification must be produced by the object-glasses. 

 These, as has been explained, are made in sets of three, having 

 different powers. The lowest power will be obtained by screwing 

 the first lens only of the lowest set upon Y ; the next by screwing 

 on the second ; and the next by screwing on the third ; by which 

 the powers of all the three will be combined. 



If it be desired to obtain a still higher power, these lenses 

 being taken off, the first lens of the set next in order is screwed 

 on, then the second, and in fine the third, by which another 

 series of three increasing powers is obtained. 



In this manner, by a suitable assortment of object-glasses and 

 eye-pieces, any desired degree of amplification can be obtained. 



The height of the case A and the length of the pillar c c are 

 so arranged, that when the case is placed upon a table of the 

 usual height, the eye of an observer of average height when seated 

 will be on a level with the eye-piece s. 



When the observer is about to submit an object to examination, 

 having mounted the instrument, placed it firmly upon a table 

 with an even surface so as to prevent any rocking or instability, 

 and regulated the height of his seat so that his eye shall be at the 

 level of the eye -piece, he selects an eye -piece and object-glasses 

 having a suitable magnifying power, and in doing this it is most 

 important to commence with a low power, to be gradually 

 increased. For this purpose, one object-glass of a set is first 

 screwed on, after which two, and in fine three, are used. 



In this manner a survey is taken of the general outline and 

 larger parts in the first instance, and the more minute parts 

 afterwards. 



78. The most generally convenient position for the instrument 

 is that which is shown in fig. 37. If a vertical position be 

 desired, it may however be easily obtained. For this pur- 

 pose the rectangular piece v is drawn out of the bayonet-joint, 

 and the object-tube is directly inserted in the body, so that its 

 axis shall be horizontal and coincident with that of the body n 

 and the eye-tube T. The body is then turned upon the joint c 

 until it is raised into the vertical position. The relative position 

 which the parts then assume is that which is shown in fig. 38. 

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