66 F. E. Wright — Oblique Illumination in 



dent beams of parallel light) and the angle of the cone of illu- 

 mination is often disregarded, but is fundamental to a clear 

 understanding of the function of the condenser in petrographic 

 microscope work. 



Still another point requires explanation. In tigs. 3 and 4 

 the eifect of closing the iris diaphragm in the lower focal plane 

 of the condenser is illustrated. This case is practically the con- 

 verse of that considered in the last paragraph. By closing the iris 

 diaphragm we decrease the angle between the emergent beams ; 

 in shore, we reduce the numerical aperture of the condenser. 

 By shifting the small opening AB as in fig. 5 we obtain a nar- 

 row bundle of obliquely emergent rays. These rays are not 

 strictly parallel but form a narrow cone whose angle of opening 

 is so small that it is negligible in microscope work. To obtain 

 obliquely incident light by this method it is best to employ a 

 sliding iris diaphragm ; but almost equally good results can be 

 obtained by using a sliding stop or metal strip in the lower 

 focal plane of the condenser ;* a good substitute for such a stop 



Fig. 6. 



* A simple device has been used by the writer and found satisfactory in 

 practice. In the lower focal plane of an aplanatic condenser (n.a. = 1*4) a 

 glass coordinate scale is inserted, which gives directly the angle of inclina- 

 tion of the emergent rays ; this scale is used primarily for the direct meas- 

 urement of optic axial angles. Directly below the scale an iris diaphragm is 

 inserted, so that its plane practically coincides with that of the glass scale. 

 Attached to tha projecting base of the condenser and supporting the iris 

 diaphragm is a collar into which a brass slider fits below the iris diaphragm; 

 this slider serves two purposes : ( 1) as a sliding stop or strip to produce 

 oblique illumination ; (2) as a frame for a large sensitive tint plate (fig. 6). 

 The supporting collar together with iris diaphragm can be rotated and allows 

 the observer to obtain obliquely incident light in any azimuth, and also to 

 insert the sensitive tint plate so that its ellipsoidal axes include any given 

 angle with the principal nicol planes. This last feature is especially impor- 

 tant for the examination of fine, weakly birefracting grains, because it ena- 

 bles the observer to rotate the sensitive tint plate to a position where its 

 ellipsoidal axes almost coincide with the principal nicol planes. The field 

 illumination resulting from the sensitive tint plate is then hardly perceptible, 

 while the change in interference tint from red to blue or yellow is clearly 

 marked in the minute mineral plate in the diagonal position. When the sen- 

 sitive tint plate is inserted in the usual diagonal position the intensity of 

 illumination of the field is so great that the delicate interference tints from 

 the minute grains are completely veiled and lost to view. These statements 

 apply with equal force to observations in convergent polarized light, espe- 

 cially to the determination of the optical character of thin., weakly birefracting 

 plates. (See F. E. Wright, Methods of Petrographic- Microscopic Research. 

 Carnegie Institution of Washington, Pub. 158, 73, 1911.) 



