THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 601 



The Object-Image. 



Under this heading I include all observations in which the 

 object itself appears in focus in the field of the microscope. 



Examination in Ordinary Light. The crystal section should 

 be brought into the centre of the field, so that it lies beneath the 

 intersection of the cross wires, and the stage rotated till the index 

 reading is zero. The outline and any other characteristic features 

 should now be traced or sketched, surrounded by a circle repre- 

 senting the margin of the field, and a scale of microns with the 

 numerical value of the magnification added. The scale is con- 

 structed with the assistance of an eye-piece micrometer calibrated 

 from a stage micrometer. The position of the cross wires is 

 shown by short radial lines drawn inwards from the circumfer- 

 ence (figs. 1-3). The right end of the right and left cross wire is 

 marked with outside the circle, because it is the direction of 

 the vibration of the nicol, when one only is inserted, and the 

 positions of the other ends of the cross wires by 90, 180 and 

 270, in the same cyclical order as the graduations on the 

 stage, which are usually contrary to those of the hands of a 

 watch. 



The stage is now rotated, and as the trace of a face, cleavage 

 or other rectilineal marking, such as a line (representing a plane) 

 of inclusions, comes into a position of parallelism with the right 

 and left cross wire, the latter should be inserted in its new position 

 in the sketch as an interrupted line across the field, and dis- 

 tinguished on its right extremity outside the circle by the 

 index reading of the stage (figs. 1-3). As each line comes twice 

 into the right and left position, it will have readings at both 

 ends, which will differ by 180. All these readings will follow 

 each other in the sketch in their cyclical order. 



Extinctions. Both nicols are now inserted in the crossed 

 position and the stage rotated. If the crystal section remain 

 dark through a complete rotation, the crystal section is either 

 isotropic or cut at right angles to the optic axis of a uniaxial 

 crystal. If it continue uniformly faintly illuminated, it is at 

 right angles to an optic axis of a biaxial crystal. Usually, 

 however, it will be dark at four points in the rotation when the 

 directions of vibration of light traversing the crystal section 



