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the eye-piece. A quartz-plate is useful in examination with polarized 3 
licht. It should be arranged between the two Nicol prisms, 
either below the stage or in the tube above the objective, so as to be | 
conveniently slipped in and out of the field as required. A nose- 
piece for two objectives screwed to the foot of the tube saves time 
and trouble by enabling the observer at once to pass from a low to a 
high power. The numerous pieces of apparatus necessary for physio- 
logical work are not needed in the examination of rocks and minerals. — 
3. Methods of Examination.—Examples of the nature of the 
kind of research practicable with the microscope in geology having 
already been abundantly given, a few hints may be here added for 
the guidance of the student in making his own microscopic — 
observations. 
Reflected Light.—It is not infrequently desirable to observe with 
the microscope 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. Hence one of the most useful 
preliminary examinations of a prepared slice is to place it in the 
field, and, throwing the mirror out of gear, to converge as strong a 
light upon it as 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 transmitted light, and 
show only their external form. But by throwing a strong light upon 
their surface we may often discover not only their distinctive colours 
but their characteristic internal structure. Titaniferous 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. But with reflected light the cleavage and lines 
of growth of the mineral can then often be clearly seen, and what 
seemed to be uniform black patches are found in many cases to 
enclose bright brassy kernels of pyrite. Magnetite also presents a — 
characteristic blue-black colour, which distinguishes it from the 
other iron oxides. 
Transmitted Lnght.—It is, of course, with the light allowed to 
pass through prepared slices that most of the microscopic examination 
of minerals and rocks is performed. A little experience will show 
the learner that in viewing objects in this way he may obtain some- 
what different results from two slices of the same rock according to’ | 
their relative thinness. In the thicker one a certain mineral or 
rock, obsidian for example, will appear perhaps brown or almost 
black, while in the other what is evidently the same mineral may be 
ale yellow, green, brown, or almost colourless. Triclinic felspars seen 
in polarized light give only a pale milky light when extremely thin, 
but present bright chromatic bands when somewhat thicker. 
Polarized Light.—By means of polarized light an exceedingly 
delicate method of investigation is made available. We use both — 
