258 



THE SIMPLE MICROSCOPE. 



a small circle, and each following one in a somewhat larger^circle, 

 until at length its diameter approximately equals that of the pupil. 

 The position of this circle corresponds with the object-point ; its 

 distance from the axis increases and decreases with the distance of 

 the latter. 



If the eye is adjusted fo: infinite distance, the section of the 



FIG. 140. 



incident cones of light consequently being equal to the principal 

 planes of the pupil, then the angle of aperture o> is obviously 

 determined for the middle of the field of view by the formula 



w d 



tan y = ,. Tf 



where d denotes the diameter of the pupil, and / the focal length. 

 Smaller angles of aperture are, therefore, approximately in inverse 

 ratio to the focal length. If d is taken at 3 mm., calculation gives, 

 for example, the following values : 



With these values coincide approximately also those which we 

 should obtain under similar assumptions for a finite distance of 

 vision of 100 250 mm. 



If there is situated beyond the last surface of the system a 

 diaphragm, the aperture of which is less than the pupil, the size 

 of this aperture evidently determines that of the incident cones 

 of light. This also holds good in a certain sense for any other 

 diaphragms inserted between the refracting surfaces ; but in this 



