628 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



hut if the mineral has about the same refractive index for 

 blue light as the medium, but a higher refractive index for 

 red light, only the red light will be obscured on the same side 

 as the finger, so that the shadow will have a bluish tinge. 

 When all the refractive indices of the mineral are included 

 between the extreme indices of the medium, there will be 

 a bluish colour on the finger side and a yellowish-red one 

 on the opposite side ; and when the red refractive indices are 

 the same but the blue refractive index of the medium is greater, 

 there will be a red shade on the far side. The colours obtained 

 with particular liquids are sometimes very characteristic of 

 minerals, and may thus be employed for their identification. 



All the particulars obtained in the investigation of the mineral 

 sections should be embodied in the sketch as shown in fig. 1. 



The following is a brief abstract of the procedure in the detailed 

 examination of the optical characters of a mineral. 



A. Examination of the object-image. 



1. With stage in the zero position, sketch the mineral and 

 indicate the positions of 0, 90, 180 and 270 by short lines 

 directed inwards from the circumference (p. 601). 



2. Determine the positions of edges, cleavages and other 

 rectilinear directions in the mineral and indicate them by dis- 

 continuous lines through the centre, each distinguished by its 

 two index readings (p. 601). 



3. Determine the extinctions or directions of vibration and 

 show them as continuous lines through the centre with index 

 readings (pp. 601-604). 



4. Note the absorption colours and other phenomena shown 

 by light vibrating in these directions (p. 604). 



5. Determine the character of the directions of vibration 

 and the amount of relative retardation (pp. 604-613). 



B. Examination of the directions-image. 



6. Determine the longitudinal direction and its proximal 

 end, and deduce the character of the section, noting at the 

 same time the nature of the movement of the isogyre and any 

 special features in the directions-image (pp. 617-624). 



All the above observations should be made, if possible, without 

 moving the object. If any movement be necessary, it should 

 be made without changing the orientation. This is best effected 

 bv the use of a mechanical stage. 



