18 



THE MICROSCOPE 



Immersion 

 flaids. 



flatness of 

 field. 



Depth of 

 focus. 



Cioarse 



focossing 



adjustment, 



be used without the cedar-wood oil because the lenses must 

 be specially constructed for the conditions under which they 

 are used. 



No immersion fluid but cedar-wood oil, or a fluid sold for the 

 purpose with exactly the same optical properties, must be used. 

 Water, paraffin, or several other substitutes which are some- 

 times inadvertently employed, entirely destroy the fine quality 

 of the image formed by an oU-immersion lens. 



In every optical instrument the centre of the field gives the 

 finest definition, and the object being examined should be placed 

 near the centre. An absolutely flat field is incompatible with 

 the finest definition in the centre, and although in certain types 

 of telescopes and photographic lenses the importance of a flat 

 field is so great that a compromise is made, no deterioration of 

 the central image can be allowed in the microscope. 



The penetration or depth of an object glass or the number of 

 different layers of an object that can be seen sharply at the 

 same time with a microscope is very small. With lenses of a 

 high aperture, and therefore in general of a high magnifying 

 power, the penetration decreases at a very rapid rate, and the 

 power of seeing different planes sharply must depend on adjusting 

 the instrument. It has been said that the depth of focus of a 

 high-power microscope is reaUy the fine focussing adjustment. 

 The fine adjustment in the hands of a skilled observer is in constant 

 motion, focussing first to one plane and then to another ; by this 

 means a perception of depth is obtained which could never be 

 given by an object glass fixed at one focus. 



The penetration of the microscope may be increased by iosert- 

 ing a stop with a small aperture immediately behind the object 

 glass, but such a method reduces the aperture and consequently 

 the detail that can be seen. It is seldom adopted except for 

 photographing certain objects where the image from the upper 

 or lower portion of the object obscures the layer being 

 photographed, or for photographing objects with compara- 

 tively coarse structure. An iris diaphragm is made that will 

 screw into the body of the microscope between it and the object 

 glass for this purpose. 



There is only one position in relation to the lenses where an 

 object can be placed to give a perfectly clear picture. This 

 position is generally called the focus, and the microscope is said 

 to be " in focus " when it is so adjusted that the object is in this 

 position. It is more convenient to effect this adjustment 

 by moving the body which carries the lenses rather than by moving 

 the object. The coarse focussing adjustment is actuated by 

 a helical rack and pinion which moves the body along a slide 

 towards or away from the object. Turning the milled head so 

 that its upper edge moves towards the observer, raises the body ; 

 away from the observer, lowers it. It is a sufficiently delicate 



