78 HAMILTON HARTRIDGE ON 



which the full aperture of the objective may be filled with a 

 uniform and well-centred cone of light ; secondly, the use of the 

 flame edge restricts the illumination to a portion of the field 

 and therefore reduces stray light, which causes less fatigue to 

 the eye than if the whole field is flooded with light. 



If the ideals of critical illumination are contrary to the Abbe 

 theory, how is it that the images which it yields are not surpassed 

 by other methods of illumination ? The only possible explanation 

 is that critical illumination has survived not in spite of, but 

 because of, its imperfections. 



Section III. 

 Alternative Methods of Illumination. 



The superiority of critical illumination being illusory, the 

 existence of other methods of illumination equal to or superior 

 to it must be looked for. 



According to the Abbe theory, the ideal position for the 

 illuminant, when a condenser is used, is at approximately the 

 lower focal plane of the condenser, for under these circumstances 

 light-rays leave the top surface of the condenser as nearly 

 parallel bundles. Such a position is an impractical one in almost 

 all cases, because of the closeness of the lower focal plane 

 to the bottom lens surface. In certain condensers the lower 

 focal plane is actually within the lower combination. I therefore 

 used the alternative method of throv/ing a magnified image of the 

 light-source into the position of the lower focal plane of the con- 

 denser by means of a bull's-eye condenser, which was itself placed 

 at the correct distance from the substage condenser. This bull's- 

 eye lens was a Petzval combination, being aplanatic and cor- 

 rected for chromatic aberration. Immediately in front of the 

 bull's-eye was mounted an iris diaphragm, so that the size of field 

 illuminated by the substage condenser could be controlled. As 

 illuminant I employed the flame of a paraffin lamp placed broad- 

 side on, so that its focused image should completely fill the 

 aperture of the substage condenser. Care was taken to cause the 

 flame image to coincide with the lower focal plane of the con- 

 denser. This adjustment was effected by first focusing an 

 auxiliary microscope on the upper focal plane of a 4-mm. Holo- 

 scope objective (the plane of the image formed by the objective 

 of distant objects being used) and then using the combination 



