TESTING THE CHROMATIC ABERRATION. 



159 



a violet or blue border, and on the left a red or orange-coloured 

 one. If, on the other hand, the chromatic aberration is over-cor- 

 rected, the left side appears violet, and the right red or orange. 



Upon this fact is based the method recommended by Mohl and 

 Harting. Let w (Fig. 94) be the self-luminous surface under 

 observation for instance, the virtual focus of an air-bubble or 

 mercury-globule. The medium focal adjustment gives a sharp and 

 generally colourless image, because the incident cones of light 



FIG. 93. 



FIG. 94. 



occupy the whole aperture of the objective, and the opposite 

 aberration-colours of the right and left half unite to form white 

 light. If we raise the focus of the Microscope to the plane M N, 

 the conditions of the refraction of the rays are essentially altered. 

 The real image now appears lower down in the body tube at ?//, 

 and the eye-piece receives only the diverging rays which are pro- 

 jected as a circle of light on the image-plane P Q. Each half of 

 the eye-piece, therefore, acts separately ; a given sector of the circle 



