QUEKETT MICROSCOPICAL CLUB. 663 



Reporter from its commencement, Mr. Alpkeus Smith, who was 

 their Honorary Librarian for forty years, and Mr. Thomas 

 Powell, whom they all had the pleasure recently of congratulating 

 upon the attainment of his eightieth birthady. He hoped they 

 would long be able to attend the meetings of the Club. 



Dr. John W. Evans, of the Geological Department of the 

 Imperial College of Science, South Kensington, then gave a 

 lecture on ' ; The Microscopical Examination of Minerals." 



Dr. Evans stated that for many reasons a stationary stage 

 and revolving nicols are most convenient, but add greatly to 

 expense, and he found high-power objectives of great use for 

 special examinations. Tube and eyepiece must be provided 

 with a slot, so that a quartz wedge, gypsum plate, or eyepiece 

 micrometer may be inserted in the focus of the eyepiece. This 

 slot should be placed in a diagonal position, and not, like in 

 many foreign instruments, from left to right, which makes 

 special wedges and plates necessary. A good rock slice should 

 range between 20 and 30 microns in thickness, but with trans- 

 parent minerals much thicker sections may be usefully em- 

 ployed. For examination the section should be brought into 

 the centre of the field, so that it lies beneath the intersection 

 of the crossed wires, and the stage rotated until the index read- 

 ing is zero. If with both nicols in the crossed position and 

 the stage rotated the crystal section remain dark through a 

 complete rotation, it 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 it will be dark at four points in the 

 rotation when the directions of vibration of light traversing 

 the crystal section are parallel to those of the nicols. These 

 four points are known as " extinctions.'' 



There is usually some difficulty in determining the position 

 of maximum darkness corresponding to the true position of 

 extinction, and one of the simplest of many methods is to rotate 

 the stage towards the position of extinction alternately from 

 opposite directions and to note the readings on each side where 

 the same degree of obscuration has been obtained. The mean 

 of several pairs of observations will give approximately the 

 true position. 



Ascertain which of the extinctions or directions of vibration 



