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



term which is to be preferred to the word " konoscope " employed 

 by some authors. 



If a microscope, from which the eye-piece had been removed, 

 were directed vertically upwards towards a cloudless sky at 

 night, the images of all the brighter stars within a certain distance 

 of the zenith, dependent on the angular aperture of the objective, 

 would be seen on the principal focal surface of the objective 

 that is to say, its focal surface for light from an infinite distance. 

 Each of these images would be formed of light which had been 

 travelling by parallel paths, or in other words in the same direc- 

 tion, which would of course be different for different stars. By 

 day the whole field would be illuminated and every point in it 

 would represent light which had reached the objective from a 

 particular direction. If a mineral section were now interposed 

 close to the objective, every illuminated point on the focal 

 surface would represent a direction in the crystal section, which 

 would be determined by the construction of the objective, the 

 position of the point relatively to it and the refraction at the 

 surface of the section. The image thus obtained representing 

 different directions in a mineral may be described as the directions- 

 image, as opposed to the object-image in which the microscope 

 is focused on the object itself. 



As it is inconvenient to direct the microscope to the sky, the 

 different directions in the mineral section are illuminated by 

 placing below the stage and above the mirror of the microscope 

 a condenser consisting of a convergent lens or system of lenses. 

 For this reason the directions-image is frequently referred to 

 as the "image in convergent light," an altogether misleading- 

 expression, since convergent light is habitually employed 

 with close objectives, when the microscope is focused on the 

 object itself, or in other words when the object-image is under 

 examination. In observing the directions-image it is usually 

 desirable to employ wide-angled objectives, so as to include 

 as many directions as possible, and the angular aperture of 

 the condenser must be at least as great. 



The directions-image of small crystals, grains and fragments 

 may be examined in like manner, though the results are modified 

 by the varying effects of refraction at different points, unless 

 the medium in which the object is immersed has approximately 

 the same index of refraction as the object itself. The inter- 



