78 F. E. Wright — Oblique Illumination in 



By observing the rise and fall of interference colors on a 

 mineral plate for light waves incident under different angles 

 and in different azimuths, it is possible to determine the crys- 

 tal system of the mineral, as Schroeder van der Kolk and 

 Fedorow were the first to show. It should be noted, however, 

 that the results obtained by this method of observation are 

 also obtainable from interference figures in convergent polar- 

 ized light. This can be clearly demonstrated by means of a 

 small movable stop in the lower focal plane of the condenser 

 (method 2 above) ; by shifting this stop, light from any point 

 in the field can be had to the exclusion of all other rays and 

 thus any particular interference color in the interference 

 figure singled out for observation. If now, without disturbing 

 the stop, the mineral plate itself be observed, its interference 

 color will be found to be identical with that noted in the inter- 

 ference figure. By moving the stop to different positions, 

 different interference tints can be obtained, but in all cases the 

 interference color observed through the stop in the interfer- 

 ence color is identical with that observed on the plate itself. 

 The reason for this relation is not far to seek. The lower 

 focal plane of the condenser is conjugate to the upper focal 

 plane of the objective while the object plane is conjugate to 

 the image plane in the eye-piece. The interference phenomena 

 take place in the upper nicol and the interference colors as we 

 observe them appear after the waves have emerged from the 

 upper nicol ; after the interference color has been once formed 

 it does not suffer further change except for the errors of the 

 lens system. It is evident, therefore, that the interference 

 color produced by a narrow beam of obliquely transmitted 

 light is practically the same, whether seen in the interference 

 figure or on the mineral plate itself. 



An excellent illustration of this phenomena can be had by 

 observing a basal section of a uniaxial mineral. In convergent 

 polarized light such a plate furnishes a black cross with concen- 

 tric isochromatic lines. If a stop with small opening be used 

 and only a narrow cone of light be allowed to pass through the 

 plate parallel to its principal axis (the central part of the inter- 

 ference figure), the plate itself will appear much darker than 

 it does when a wider cone of light is employed. By shifting 

 the stop in one of the diametrical planes parallel with the 

 principal nicol planes (i. e. parallel with one of the dark bars 

 in the uniaxial interference figure) the field preserves its dark- 

 ness ; but if the stop be shifted along a diagonal diametral 

 plane so that in the interference figure the small aperture 

 appears in the light quadrants between the dark bars, the plate 

 itself becomes noticeably illuminated, thus giving rise to the 

 dull gray interference tints characteristic of most sections of 



