xvn MICROSCOPICAL INVESTIGATION 347 



le by enabling the observer at once to pass from a 

 low to a high power. Two Nicol prisms are indis- 

 pensable ; one of theae is to be fitted below the stage, 

 the other is moat advantageously placed over the eye- 

 piece. A quartx- plate is useful. It should be to 

 arranged as to be conveniently slipped in and out of the 

 field as required. The numerous small pieces of appar- 

 atus necessary for physiological work arc not needed in 

 examination of rocks and minerals, 

 R(fltt ti^k*, It is always desirable to observe the 

 characters of a rock as an opaque object This cannot 

 usually be done with a broken fragment of the stone, 

 except of course with very low powers. But it may be 

 accomplished by making use of the powder as explained 

 on p. 236. Another method is to place a thin slide of 

 the rock in the field of the microscope, and throwing the 

 mirror out of gear, to converge as strong a light upon it 

 aa can be had, short of bright direct sunlight The 

 advantage of this method is more particularly noticeable 

 in the case of opaque minerals. The sulphides and iron 

 oxides, so abundant in rocks, appear as densely black 

 objects with transn lit, and show only their ex- 



ternal form. Hut by throwing a strong light upon their 

 surface we may often discover that they possess a distinct 

 haracteristk internal struct aniferous iron 



is an admirable example of the advantage of this method. 

 Seen with transmitted light that mineral appears in black, 

 utterly structureless grains or opaque patches, though 

 frequently bounded by definite lines and angles. 

 with reflected light the cleavage and lines of growth 

 of the mineral can often be clearly seen, and what 



