of Minerals in the Thin Section. 



321 



since by its use all crystal faces are reduced to points and zones 

 to straight lines. 



In the orthographic (also called orthogonal or parallel or 

 ocular) projection, the eye of the observer is supposed to be at 

 an infinite distance above the plane of projection and to look 

 directly down upon the sphere. The lines of sight are then 

 parallel and the points on the sphere are vertically above their 

 projection points on the central diametral plane (tigs. 2 and 3). 

 In this projection, great circles appear as ellipses and small 

 circles as straight lines. This projection is especially impor- 



In fig. 3 the point P of the sphere, which is located in this case by 

 the intersection of the great circle ATP and the small circle DPK, 

 in the orthographic projection F and is there located by the ellipse AHF, 

 the orthographic projection of ATP, and the straight line DFL, the 

 projection of DPK. F is also the point of intersection of the diametral 

 plane CGB with the line PF, normal through P to that plane. 



tant since all interference phenomena observed in convergent 

 polarized light under the microscope appear to the eye of the 

 observer as they would were the actual interference phenomena 

 plotted in this projection. The serious drawback to its general 

 application in optical work lies in the rapid decrease of its 

 sensitiveness to differences in angular distances near its outer 

 margin. The interference figures below are all represented in 

 orthographic projection, although their construction was 

 accomplished in part by using the stereographic projection. 

 In fig. 1, the ellipses represent great circles with a common 

 diameter of intersection and drawn at intervals of 2° apart ; 

 while the straight lines are the projections of small circles, also 

 2° apart and corresponding to small circles of latitude. As on 

 the sphere itself, the angular distance between any two points 

 in projection can be found by passing through the two points 

 the common great circle (ellipse in projection) and counting 

 directly the distance in degrees by means of the small circles. 



