MICROSCOPES. 205 



shows the importance of regulating the aperture of the con- 

 densing lens used for illumination, since with one aperture 

 the edge of the crystal would be shown by a dark band, and 

 in the other might be invisible. When low powers are used 

 the most convenient condenser is a moderately large plano- 

 convex lens of short focal length, or two so combined as to 

 give less aberration, but in using high powers an achromatic 

 condenser of more complex structure, short focus and 

 large aperture, is very desirable. Even with rough and im- 

 perfect mechanical arrangements I have been able to see 

 sufficient to convince me that even with achromatic con- 

 densers it is a very great advantage to have two different 

 diaphragms, one to modify the divergence of the light, and 

 the other the size of the beam passing up through the sub- 

 stage. By using only light of high angle of divergence, and 

 a small opening in the sub-stage diaphragm, some objects are 

 seen to very great advantage. Such an illumination is im- 

 possible with condensers constructed in the ordinary manner, 

 with only one diaphragm placed in such a position that it 

 gives imperfectly both these effects combined. 



You must remember that the light passing near the centre 

 of a condenser is inclined at a small angle to the line of 

 vision, whilst that which passes through the exterior zone 

 is much more divergent. Hence the light from the outside 

 part of the condenser may pass through certain parts of the 

 object, but if you stop off the outer zone of the condenser 

 no light may pass through those parts and you may see 

 a well defined dark edge in such a crystal as I have alluded 

 to. By making the opening wider and wider, and thus 

 allowing more and more divergent light to pass to the 

 object, you may as it were obliterate the crystals on the 

 stage. It is sometimes important thus to be able to make 

 the light pass through an inclined edge. For instance, I 

 have a beautiful crystal of sapphire with a fluid cavity con- 

 taining liquid carbonic acid. It happens to lie in such a 

 position in the crystal that if I use light of a moderate angle 

 of divergence, the fluid cavity is completely hidden, because 

 no light passes through where it lies, but on increasing the 

 aperture and the obliquity of the light the dark shadow dis- 

 appears, and the fluid cavity is perfectly well seen by trans- 

 mitted light. The possibility of either seeing the object or 

 not seeing it at all, thus entirely depends on knowing 



