Notices of Memoirs — Directions Image of a Mineral. 47 3 



Similarly, the two orders of exogenetic forms are each subdivided 

 into nine families — 



Family 1. Forms due to the action of the run-off. 



2. ,, ,, ,, percolating water. 



3. ,, ,, ,, streams and rivers. 



4. ,, ,, ,, life. 



5. ,, ,, ,, lightning. 



6. ,, ,, ,, sun heat. 



7. ,, ,, ,, the atmosphere. 



8. ,, ,, ,, frozen water. 



9. ,, ,, ,, the sea. 



Each family is then subdivided into genera and species or specific 

 forms. It is suggested that a land form be defined as any surface or 

 slope which may be referred in origin to the operation of a single 

 process or force. Monodynamic surfaces of this kind being rare, 

 however, the commoner polydynamic surfaces may be classified 

 according to the predominant force amongst those responsible for the 

 production of the surface. This definition may be extended to- 

 include such surface features as cones or domes enclosed by one 

 continuous surface, or such features as ridges or mounts enclosed by 

 surfaces meeting in edges, provided that all these surfaces may be 

 classified as examples of the same specific form. 



(7) The Isolation op the Directions Image of a Mineral in a Rock- 



slice. By J. W. Evans, D.Sc, LL.B. 

 TI^HE author discussed the different methods by which the inter- 

 _L ference figures of a small mineral in a rock-slice may be kept 

 distinct from those of adjoining minerals. He recommended two. 

 In one, which he believes to be new, a diaphragm with a small 

 aperture is placed below the condenser, which is lowered till the 

 image of the aperture appears in focus on the rock-slice. In some 

 microscopes the iris diaphragm provided for the Becke method of 

 determining the refractive index may be employed. In others it is 

 too near the condenser. The aperture should be sufficiently large to- 

 illuminate the maximum area of the mineral under investigation, but 

 no portion of the others. The directions image may then be observed 

 in any of the usual ways. Unless the condenser and diaphragm 

 revolve with the stage the aperture must be very carefully centred 

 with the axis of rotation. 



The other method was proposed by Becke in 1895, but is very little 

 known. The diaphragm is placed in the focus of the eye-piece so as 

 to shut out all except the mineral selected. The Becke lens, or 

 system of lenses resembling an eye-piece, is placed above the eye- 

 piece, when the directions image of the mineral will be seen without 

 any admixture of light from its neighbours. This method has the 

 advantage that the diaphragm is less highly magnified at the time of 

 adjustment. When a rotating stage is employed, a very accurate 

 centreing of the nose-piece of the microscope is required, so that the 

 coincidence of the object with the aperture may be maintained. 



The common practice of placing a diaphragm for this purpose 

 immediately below the Bertrand lens rests on no scientific basis, and 

 is not effective in shutting out the light of minerals other than that 

 ■which is being studied. 



