Illumination and Illuminating Apparatus 93 



exactly obtained. It will be found of utility to have the arc, 

 through which the lever controlling the iris diaphragm 

 travels, provided with a scale of divisions, so that results 

 may be quickly reproduced or any special aperture may be 

 obtained ; but for this purpose it is necessary that the 

 diaphragm shall respond immediately on the pressure of the 

 lever handle ; there must be no loss of time in the move- 

 ment. The carrier is further provided with an arm, having 

 a rotating cell, in which may be placed stops for producing 

 dark ground illumination in the same manner as with the 

 spot lens, described on page 101 ; also stops for obtaining 

 oblique illumination for the resolution of the markings on 

 diatomaceae, and for holding tinted glasses or light screens. 

 This form of carrier is applicable also to the Abbe 

 illuminator. 



A very efficient condenser can be oftentimes formed by 

 fitting a low-power objective into a suitable carrier in the 

 sub-stage. A small iris diaphragm, called the Davis's 

 Shutter, having the universal male Society's screw at one 

 end and a female screw at the other, together with a tube, 

 fitting into the sub-stage and provided with the universal 

 thread, into which the iris diaphragm may be fitted, is a 

 very suitable carrier for the objective. 



The Aplanatic Aperture of a sub-stage condenser is 

 ascertained in the following manner : 



The condenser is accurately centred, and both it and an 

 objective are focused in the usual way on an object mounted 

 in Canada balsam, the edge of the lamp-flame being employed. 

 We will presume that the objective has a numerical 

 aperture of '5 N.A. The full aperture of the condenser is 

 then used, and the object so placed that the balsam portion 

 is still between the condenser and objective, but the object 

 itself is not in the field. It would be well that a balsam- 

 mounted object were always used for this experiment, as 

 the result is slightly affected by different media. The 

 eyepiece is now removed, and the back lens of the objective 

 is examined. It may be found that it is completely filled 



