322 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



condenser and oil-immersion. It will be noticed that the first dia- 

 phragm in fig. 43 bears the figures * 85 and 1 • 33 on its central and 

 peripheral portions respectively. These figures denote the range of 

 apertures for which the condenser is serviceable. With this particular 

 diaphragm, therefore, an objective must be used whose aperture does 

 not exceed 0'85. With stronger objectives the central area of the 

 diaphragm must be enlarged, and this is done by superposing one of 

 the other three stops on the first one, the little hole in the disk fitting 

 on to the pin shown in the centre of the left-hand one. The figures 

 on each denote the aperture of the corresponding objective. 



The best light source for dark ground illumination is a specially 

 constructed small arc-lamp, with carbons mutually perpendicular. It 

 requires a current strength of 4 amperes and can be obtained by plug- 

 contact from a domestic installation. The light from this lamp is 



Fig. 43. 



parallelized by a lens, and then directed on the plane mirror of the 

 Microscope (fig. 44). A hinge on the vertical lamp pillar makes it 

 possible to slope the lamp so that the Microscope mirror is completely 

 filled with light. In the use of dry systems a ground-glass disk should 

 be inserted between lamp and mirror. This has the effect of deepening 

 the contrast between dark ground and bright object. With weak ob- 

 jectives, i.e. up to about aperture 0*4, the condenser should be used 

 dry ; but with stronger objectives it should be used as an immersion 

 condenser, and water will usually be found to be a satisfactory medium. 



In order to effect quickly a change from dark-ground to light- 

 ground illumination, or vice versa, a special central stop arrangement 

 can be inserted under the iris-carrier of the condenser, and can be quickly 

 slid into and out of action. 



The above described condenser can also be used as an apertometer, 

 as will be understood from the following. The effect of the proximity 

 of the iris to the lower focal plane of the condenser is to throw back the 

 image of the iris projected by the condenser almost to infinity. Of this 

 image the objective projects an image close to its lower focal point ap- 

 proximately near to the extreme lens surface facing the ocular. The 

 rays of a beam entering the condenser parallel to the optic axis com- 

 bine at the front focal point of the condenser ; inasmuch as this point 

 practically coincides with the lower focus of the objective, the light-cone 



