MICROSCOiPIC ILLUMINATION. 



81 



the required resolution of lines or dots was obtained. The 

 diaphragms were now gently closed till resolution was seen to 

 break down in some part of the field ; this was found to happen 

 more quickly when the Davis diaphragm was closed than was 

 the case with the substage iris. 



Diagram 1. 



MeTHao 2 Method 3 



Methoo4 . 



Diagram to show the Four Alternative Methods of 

 Illumination referred to in this Paper. 



A = A plane co-ordinate with slide. 

 B = A plane co-ordinate with upper 



focal plane of objective. 

 C = Substage condenser. 



I = Iris diaphragm. 



P = Petzval " bull's-eye " condenser. 



S = Source of light. 



When the appearance had been carefully noted, the other 

 illuminants were substituted. 



Any improvement in resolving power would be at once shown 

 by the appearance of structure in those parts of the field where 

 the resolution had broken down owing to the closure of the iris. 

 Any decrement, on the other hand, would have shown itself by 

 a further breakdown of the image. 



In the case of the resolution of Amphipleura pellucida a specimen 

 mounted in realgar was employed. It was found that great care 

 had to be taken to eliminate spherical aberration from the 

 images of both condenser and objective. Further it was found 

 advantageous to use an eyepiece with a small field of view. It 



