6o 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
3 or 4 mm. focal length, grains o.oi to 0.02 mm. in diameter and either in 
the powder or the ordinary thin section, furnish good interference figures 
which ordinarily would be completely overshadowed and not discernible if 
the adjacent light were not excluded. 
(5) A large substage condenser is used together with a large Thompson 
or Ahrens prism in place of the usual nicol and condenser with removable 
upper lens. This arrangement, which was first introduced by Leiss on the 
Fuess microscope, is a marked improvement over the usual arrangement, 
as it does away with the more or less complicated devices for removing 
the upper condenser lens from the optic axis of the microscope. 
(6) The objective clamps and supporting rings are made of casehardened 
steel, and not of brass, which is soft and too easily indented to hold its 
surfaces true for any length of time. 
FIG. 39. 
(7) The mechanical stage is new in design (Plate i, Fig. 3), is practically 
dust proof, has a play of 24 mm., and is mechanically simple in construction. 
By means of the screws 7/i and 7/ 2 , of half millimeter pitch and with gradu- 
ated heads, movements of o.oi mm. can be read off directly. 
(8) The distance between the stage and the arm of the draw-tube support 
is sufficient to allow the use of the universal stage. 
(9) In this instrument the axis of both the draw-tube and the substage 
condenser support coincide precisely with that of the rotating stage. Since 
the ocular and the condenser remain automatically centered with respect 
to the rotating stage, while the objective changes its position slightly on 
each insertion, the usual centering screws (ai oj, Plate i, Fig. 3) for the 
objective itself have been introduced. The direction of motion of such 
centering screws should be parallel with the cross-hairs of the ocular, as the 
