20 MICROSCOPE AND ACCESSORIES. 



fluid is put between the slide and condenser, forming the so-called 

 immersion illuminator. This is necessary only with objectives of 

 high power and large aperture or for dark-ground illumination. 



Centering the Illuminator. — The illuminator should be centered to 

 the optic axis of the microscope, that is the optic axis of the condenser 

 and of the microscope should coincide. If one has a pin-hole dia- 

 phragm to put over the end of the condenser (Fig. 20) — that is a dia- 

 phragm with a small central hole — the central opening should appear 

 to be in the middle of the field of the microscope. If it does not, the 

 condenser should be moved from side to side by loosening the center- 

 ing screws until it is in the center of the field. In case no pin-hole 

 diaphragm accompanies the condenser, one may put a very small drop 

 of ink, as from a pen-point, on the center of the upper lens and look at 

 it with the microscope to see if it is in the center of the field. If it 

 is not, the condenser should be adjusted until it is. The microscope 

 and illuminator axes may not be entirely coincident even when the cen- 

 ter of the upper lens appears in the center of the field, as there may be 

 some lateral tilting of the condenser, but the above is the best the ordi- 

 nary worker can do, and unless the mechanical arrangements of the 

 illuminator are very deficient, it will be very nearly if not absolutely 

 centered. , 



§ 56. Mirror and Light for the Abbe Illuminator. — It is best to 

 use daylight for this as for all other means of illumination. The rays 

 of daylight are practically parallel, and it is best, therefore, to employ 

 the plane mirror for all but the lowest powers. If low powers are used 

 the whole field might not be illuminated with the plane mirror and the 

 condenser close to the object ; furthermore, the image of the window 

 frame, objects outside the building, as trees, etc., would appear with 

 unpleasant distinctness in the field of the microscope. To overcome 

 these defects, one can lower the condenser and thus light the object 

 with a diverging cone of light, or use the concave mirror and attain 

 the same end when the condenser is close to the object (PL III, 

 Fig. 20). 



§ 57. Lamplight. — If one must use lamplight, it is recommended 

 that a large condensing lens be placed in such a position between the 

 light and the mirror that a picture of the lamp flame is thrown upon the 

 mirror. If one does not have a condensing lens the concave mirror may 

 be used to render the rays less divergent. It may be necessary to lower 

 the illuminator somewhat in order to illuminate the object in its focus. 



ABBE ILLUMINATOR : EXPERIMENTS- 



§ 58. Abbe Illuminator, Axial and Oblique Light. — Use a dia- 

 phragm a little larger than the front lens of the }4 (3 mm.) or No. 7 



