_The Stage._--In the instrument we are now describing this is circular with a hole in the middle, and is so arranged as to revolve horizontally on a collar about an axis, the centre of which comes exactly underneath the centre of the objective. In other words, a straight line drawn through the eye-piece down the centre of the barrel of the microscope, and passing through the objective passes through that axis. To assist in more accurately centreing than is otherwise possible (depending on the lenses) with this cheap form of instrument, a collar with adjustable screws is ordinarily affixed to the lower part of the barrel of the microscope. The stage, with suitable clips to hold the object to be examined, is graduated so that on its being revolved it is easy to ascertain the number of degrees, at any period of the revolution, through which it has been turned. Thus, it will be observed that the object revolves with the stage. A pointer is placed in a suitable position on the frame of the microscope to facilitate the observation. _The Polariscope._--This is an indispensable adjunct, for determinative purposes it is often necessary to observe the object in polarised light. Briefly, the polariscope consists of two parts--the analyser, placed in the barrel of the microscope above the objective, and the polariser, arranged underneath the revolving stage. The analyser is so fitted that it may be shot in and out of the barrel in order that the polariser alone may be used, or the latter may be removed, leaving only the analyser in position, or both may be removed to enable the object to be examined in ordinary light, either reflected or transmitted. The lower nicol[14] is made to revolve, and the collar in which it is fixed is broadly graduated and furnished with a pointer. _Reflector._--An ordinary reversible and adjustable reflector is arranged beneath all. _Accessories._--For the more accurate determination of minerals, a quartz wedge, a quartz plate, etc., are used by the petrologist, but the description of these is beyond the scope of the present work. For examination in reflected light it is highly desirable to have a “bull’s-eye” condenser. An ordinary microscope with a revolving stage may be readily converted to petrological purposes, though it is better to have a special instrument. * * * * * The object to be examined may be in the form of (_a_) a fragment of the brick, or (_b_) a very thin slice of the same.