82 THE AMERICAN MONTHLY [April, 



whicli the ])eam of light traversing it is brought to a focus on the 

 object. If parallel rays are reflected from the plane mirror to it. 

 they will be focussed only a few millimeters above the upper lens 

 of the illuminator; consequently the illuminator should be about 

 on the level of the top of the stage, and therefore almost in contact 

 with the lower surface of the slide. For some purposes, when it 

 is desirable to avoid the loss of light by reflection or refraction, a 

 drop of water or homogeneous immersion fluid is put between 

 the slide and condenser, forming the so-called immersion ilhnni- 

 nator. This is necessary only with objectives of high power and 

 large aperture or for dark-ground illumination. 



Centring the Illuminator. — The illuminator should be cen- 

 tred 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 diaphragm to put over the end of the condenser (fig. 

 3o) — that is a diaphragm with a small central hole — the central 

 opening should appear to be in the middle of the field of the mi- 

 croscope. If it does not, the condenser should be moved from 

 side to side by loosening the centring screws until it is in the 

 centre 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 centre of the upper lens and look at it with the 

 microscope to see if it is in the centre 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 

 centre of the vipper lens appears in the centre of the field, as there 

 may be some lateral tilting of the condenser, but the above is the 

 best the ordinary worker can do, and unless the mechanical ar- 

 rangements of the illuminator are very deficient, it will be very 

 nearly if not absolutely centred. 



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 ; further- 

 more, the image of the window-frame, objects outside the build- 

 ing, as trees, etc., would appear with unpleasant distinctness in 

 the field of the microscope. To overcome these defects, one can 

 lower the condenser ahcl 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 (fig. 20) . 



Lamp-light. — If one must use lamp-light, 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. 



