134 JOURNAL OF THE [June, 



examination of the " critical point " of the contained liquid is to 

 be made, by the method and with the immersion-apparatus to be 

 described presently, it will be of advantage to mount them on 

 slides of the thinnest material and shorter than those generally 

 used by lithologists (45 mm. x 26 mm.), in order to diminish the 

 diameter of the tank and the volume of the water employed in 

 that apparatus. It will be well to mark such slides with a 

 diamond point in place of using a paper label. 



The selection of objectives for the study of these sections is 

 not difficult. Although high powers are necessary to reveal 

 some of the phenomena connected with the fluid-cavities, very 

 low powers will answer for most purposes. Ordinarily, a power 

 magnifying four hundred diameters is the most useful. Lenses 

 ranging from a one-fifth to a one-eighth have been the most 

 useful to me. Yet there are times when a power as low as that 

 of a half-inch objective can be used, even for the study of the 

 minute cavities in which there are moving bubbles. Of course, 

 the very highest powers are often desirable in examining other 

 cavities, with the limitation already expressed, that the cavity 

 must be so near the surface as to be within the focal distance of 

 the objective. 



Transmitted light is, of course, requisite for the study of these 

 cavities, but reflected light or that produced by dark ground 

 illumination is sometimes the most ready means for their first 

 discovery in running over a thin section of rock containing 

 grains of quartz. Most cavities contain more or less gas, and 

 this causes them to reflect the light like drops of molten silver 

 and to catch the eye far more quickly, often, than if they had 

 been first examined in the ordinary way. 



The thorough examination of the more minute cavities will 

 often require a command of all the resources to be found in the 

 substage condenser, the use of the diaphragms, and the position 

 of the mirror, for modifying the intensity and obliquity of trans- 

 mitted light. The variety of forms and of refractive appear- 

 ances presented in the fluid contents of the cavities, the effects 

 of the irregular shapes and projections of the cavities and of the 

 reflection of light from their walls, and the curious images pro- 

 duced by the irregular distribution and aggregation of other 

 cavities in different focal planes, often cause perplexing phenom- 

 ena, which are likely to mislead the judgment. 



