ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 35 



raise the substage as far as it will go; insert a cross-hair eyepiece 

 in the body tube and focus with a very low power upon the 

 diaphragm opening. The diaphragm opening should fall at the 

 center of the field of view directly under the cross-hairs, con- 

 centric with their point of intersection. If the image of the 

 opening is not centrally located there is something faulty in the 

 construction of the condenser or in its attachment to the sub- 

 stage, or in the alignment of objective and ocular. 



If the condenser has been found centered, we may change to a 

 high power objective and be reasonably sure that the condenser 

 will be centered with respect to the objective, providing a re- 

 volving nose-piece is not in use; but if the objective is attached 

 to an ordinary nose-piece, turning from one objective to another 

 usually necessitates a readjustment of the condenser. With high 

 powers, centering, as described above, is impossible and it will 

 be found simpler to remove the ocular and hold a tripod or 

 pocket magnifier over the tube; the image of the diaphragm 

 opening is then easily seen and its relative position ascertained. 



In testing for proper centering it is important that the mirror 

 be so placed as to yield exactly axial light. This may be assured 

 by swinging the condenser to one side and placing upon the 

 stage a preparation consisting of thin gum beaten up until full 

 of air bubbles; a very tiny air bubble is selected and brought 

 to the center of the field, it appears as a bright spot surrounded 

 by a black ring; the bubble is sharply focused and the mirror 

 adjusted by proper tipping until the bright spot appears exactly 

 at the center of the circular black ring. The light is now exactly 

 axial. This method of assuring absolutely axial light l is the 

 simplest and surest available. 



Without touching the preparation or the mirror, carefully 

 swing the condenser back in place, raise it about halfway and 

 slowly raise and lower the body tube by means of the coarse 

 adjustment, closely observing at the same time the appearance 

 of the bubble image. If the light still remains axial with the 

 condenser in place there will be no appreciable swaying of the 

 image and no change of position of the bright spot of light. If 

 1 Gage, The Microscope, p. 48, loth Ed., Ithaca, 1908. 



