CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 



233 



to be different from what it would be with the main condenser only ; 

 or (2) to make the aperture of the illuminating cone correspond 

 with that of the objective. 



The positional reason (i) can only have weight when combined 

 apparatus is used, that is, when a magic lantern objective as well 

 as microscopic objectives are used without changing the distance 

 between the main condenser of the microscope or the magic lantern 

 objective. 



FIG. 



130. OCULAR MICROMETER WITH MOVABLE SCALE. 

 (Cut loaned by the Spencer Lens Co.). 



This is a Huygcnian ocular with a 5 mm. scale divided into twenty K" mm. 

 intervals. The pitch of the screw moving the scale is Y\ mm., therefore one 

 complete revolution of the drum moves the scale one interval or '--4 mm. The 

 drum is divided into 100 graduations thus enabling one to measure looth of 

 an interval on the micrometer scale. This ocular micrometer combines the 

 advantages of the ocular micrometer with fixed scale and the filar micrometer. 

 To complete the measurement of an object not exactly between any two 

 micrometer lines the drum need be revolved only partly around. 



With reference to the aperture (2) it is one of the fundamental 

 laws of microscopic vision that the brilliancy and clearness of 

 details depend largely upon the aperture of the light which illumin- 

 ates the object, and which passes through the objective to form the 

 retinal or the screen image. As the numerical aperture of objec- 

 tives varies greatly it is necessary, if the clearest and most brilliant 

 images are to be produced, to light the object with a numerical 

 aperture equal to that of the objective. Where substage con- 

 densers are used arrangements must be made for this. 



