206 LECTURES TO SCIENCE TEACHERS. 



how to modify the angle of divergence of the light used for 

 illumination. If you had light passing through parallel, like 

 that reflected from an ordinary direct mirror, you would see a 

 broad dark band, and would not have the remotest idea that 

 such an interesting cavity existed ; but if you used as a con- 

 denser, an almost hemispherical lens, you would then see the 

 fluid cavity to perfection. The use of such simple single or 

 compound condensing lenses of large aperture is not common, 

 but they enable us to study certain objects in a very satis- 

 factory manner, since with appropriate stops the angle of 

 divergence of the light is so very much under control. This 

 principle may also be brought to bear in another class of 

 objects. There may co-exist two perfectly independent 

 kinds of structure, which with ordinary illumination may so 

 far unite together as to produce a general appearance of a 

 very misleading character ; but by varying the divergence of 

 the light or the size of the sub-stage opening, first one and 

 then the other structure may be made invisible, whilst the 

 other is seen to great advantage by itself. 



Another interesting illustration of the importance of the 

 angle of divergence of the light is furnished by the little 

 spherical cavities met with in amber, some filled with gas 

 and some filled with water. By proper illumination you can 

 see these very well, but I find that the character of the object 

 and the illumination depends on a set of conditions which 

 I believe have not attracted any attention. In looking at 

 such spherical cavities in minerals, which I choose as a type 

 because they are so simple, and we can understand them 

 perfectly well, or in looking at, a spherical cavity or a bubble 

 in glass, you see first a black outline and then a white bright 

 centre. This bright centre is nothing more than the image 

 of the opening at the bottom of the condenser seen out of 

 focus, and by a little modification in the focus you can see 

 very distinctly anything special in its character. My 

 meaning will be better understood by giving a few illustrations 

 of what may be seen in examining the gas cavities, which are 

 so extremely numerous in some specimens of amber. When 

 the opening in the sub-stage diaphragm is large, you see a 

 broad clear space and a relatively narrow dark band, extending 

 inwards from the external outline of the cavity, as shown by 

 Fig. 3 ; but on making the opening in the diaphragm small, 

 this black zone closes in, and there is only a small bright 





