30 



THE MICEOSCOPE 



Effect of 

 focussing a 

 Bubstage 

 condenser. 



light appearing as a uniform circular disc; closing the dia- 

 phragm will at first not make any change in the appearance, 

 because the condenser is co verging upon the object a beam of 

 light of greater angle than can be collected by the object glass, 

 and it will receive the full illumination until the light from the 

 condenser becomes smaller in angle than the aperture of the object 

 glass. If the diaphragm be now closed to its fullest extent, the 

 back lens of the object glass shows a small spot of brilliant light 

 in the centre ; and as the aperture of the iris diaphragm is slowly 

 opened, the spot of light slowly increases in size until the whole 

 of the back lens of the object glass is completely filled with 

 uniform light. As soon as this is done the whole of the aperture 

 of the lens is receiving direct light from the condenser shining 

 through the object as shown in Fig. 16. 



While the microscope is in this condition, the effect on the 

 appearance of the back lens of the object glass, which is produced 

 by putting the condenser in and out of focus, should be 

 observed. The iris diaphragm being open to the full extent, it 

 will be found that unless the condenser is in correct focus the 

 whole area of the back lens will not be equally filled with light, 

 and from the appearance so observed it can be realised why an 

 uncorrected condenser like the Abbe condenser cannot readily 

 be made to fill the whole area of the object glass with uniform 

 light, the reason being that the light of difierent obliquity is 

 brought to a focus at different positions. It may be that only 

 one ring of light at the edge of the object glass, or a small area 

 in the centre, or a combination of both, is being illuminated. 



The importance of the character of the illumination has been 

 ufmninatSig referred to, and the question arises as to what aperture cone of 

 light it is best to use. A well-corrected substage condenser 

 centred and in focus gives the means of completely controlling 

 this, and from what has so far been said it might be supposed 

 that the full aperture of the object glass should be always illu- 

 minated ; this is not necessarily the case, further research is 

 required before definite rules can be laid down to meet all condi- 

 tions. For the best resolution the diaphragm in the condenser 

 should never be opened to admit a larger angle of light than that 

 of the object glass, or " glare " may produce a misty appearance 

 which will destroy the crispness of the image. 



This can be avoided by reducing the size of the diaphragm 

 until the aperture just becomes visible at the edge of the back 

 lens of the object glass. To what extent the angle of light 

 admitted by the condenser should be smaller than the aperture 

 of the object glass depends upon the nature of the object. Full 

 resolution will only be obtained if the whole aperture of the 

 object glass is transmitting light, for reasons previously explained ; 

 and if the light from the condenser were not filling the aperture 

 of the object glass, and there were no object on the stage, the 



Best 



cone 



