506 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



diaphragm below the stage in the position in which the iris-diaphragm- 

 is usually placed, so that when the Bertrand or Beck lens is inserted or 

 the eye-piece removed it is seen in focus in the directions-image. After 

 the diaphragm has been adjusted as desired, the Bertrand or Beck lens 

 is removed or the eye-piece replaced, and the object-image is seen, 

 illuminated only by the vibrations which it is desired to investigate. 



If the Bertrand lens be employed to form a directions-image in the' 

 focus of the eye-piece, the diaphragm may be placed in that position, and 

 the object-image, formed above the eye-piece, may then be observed with 

 a suitable lens. This image is, however, in the ordinary microscope too 

 high above the eye-piece to be seen with a fixed lens such as the Beck 

 lens without an extra tube, and it is small and ill-defined." 



The chief objection to these methods of isolating a particular 

 direction of vibration is the great loss of light which they involve, 

 causing the illumination to be comparatively feeble even with a powerful 

 source of light. 



Where, however, a thin section is cut at right angles to a plane of 

 optical symmetry, or, what comes to the same thing, a grain is sO' 

 orientated that a plane of optical symmetry is at right angles to the 

 microscope stage, the direction of vibration at right angles to the plane 

 of optical symmetry may be isolated by means of a diaphragm with a 

 tine slit instead of a small perforation. The slit may be adjustable in 

 the same manner as that of a spectroscope, or the diaphragm may be 

 formed by a metal slide with slits of different widths, which can be used 

 as desired. 



The slit is placed in a position of parallelism with the plane of 

 optical symmetry, with the result that all the paths of the light which 

 passes it and illuminates the portion of the object in the axis of the 

 microscope are approximately in that plane. It must therefore, what- 

 ever path it follows, have one of its directions of vibration practically 

 normal to the plane in question, and the other in the plane and at right 

 angles to its path. If now a Nicol's prism be inserted in the microscope 

 in such a position that the vibrations of the light which traverses it are 

 at right angles to the plane of optical symmetry, the vibrations in that 

 plane will be eliminated, and the image will be illuminated only by 

 vibrations at right angles to it, which have, of course, all the same, 

 refractive index, so tliat definite results will be obtained. 



If the section is at right angles to two planes of optical symmetry, 

 or, what comes to the same thing, if two such planes are parallel to the 

 axis of the microscope, each may in turn be placed parallel to the slit, 

 and thus two indices of refraction may be determined. These will be 

 two out of the three principal indices of refraction, that is to say, those 

 of the three lines or axes of optical symmetry. 



In the case of uniaxial crystals, the index of refraction of the 



ordinary or basal vibrations may be observed in any section, while in a 



section parallel to the axis of optical symmetry, those of both the 



ordinary and the extraordinary axial vibrations may be determined. 



These methods are applicable to the Chaulnes procedure for deter- 



* J. W. Evans, "The Isolation of the Directions-image of Small ObjectSi" 

 Mineralogical IMag., xviii. (1916) p. 45. 



