USE AND MANIPULATION OF MICROSCOPE 167 



A. ' 

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obtain when the condenser is filled with a parallel beam DD, 

 and is exactly focussed on the object lying on the slip AA. If 

 the condenser is one with a full aplanatic cone that is, one 

 which is practically free from spherical aberration the whole, 

 or nearly the whole, of the light will be focussed on the object. 



In the case where the condenser is simply an uncorrected 

 lens or is of the ordinary Abbe illuminator type, then the 

 central rays will be focussed in a different 

 plane to the marginal rays (Fig 59) ; and to /\ 



get a clearly defined bright image of the / \ 



radiant it will be necessary to close the iris- 

 diaphragm somewhat, irrespective of what may 

 be necessary to obtain a bright image of the 

 object. 



Not only is it impossible to obtain such a 

 bright and clear image of the illuminant, but 

 there is also a general loss of light owing to so 

 much of the aperture of the condenser being 

 used in condensing the light to a point where 

 it is not utilisable by the objective. It 

 must also be remembered that unless critical 

 illumination is accurately obtained, the effective 

 aperture of the condenser itself is reduced ; 

 that is, if the condenser is adjusted so that 

 its focal point is either above or below the 

 object, the effective aperture as may be 

 observed by looking down the microscope tube FlG - 59. Diagram 



.,, , ,-, . T , A-U of Path of Rays 



without the ocular is less in proportion as the in gub _ sta g e 

 correct conditions are departed from. Fig. 60 Condenser, 

 may help to explain this. The points A, B, 

 and C may be taken to represent the position of an object in 

 the cone of light formed by the condenser. Only at A does 

 the object receive the benefit of the full cone, and at B or C 

 neither the full amount of light nor the full aperture of the 

 condenser is utilised. 



A further advantage of the large aplanatic cone becomes 

 evident when test- diatoms or similar objects have to be photo- 

 graphed with only a portion of the solid cone that is, with 

 annular illumination. In this case only a portion of the 

 marginal rays transmitted by the condenser is utilised. The 



