70 F. E. Wright — Oblique Illumination in 



hold. The angle of inclination of the rays within the draw 

 tube is so small (less than 3°) that it is difficult to place the 

 stop in a position to show the phenomena clearly. The edge of 

 the shadow thus obtained is furthermore very narrow. In case a 

 low power objective be used, the object itself is usually illumin- 

 ated by a cone of rays whose angular aperture is so large that 

 they cannot all enter the objective ; they produce, however, 

 phenomena of refraction in the object which are readily imaged 

 through the objective but which cannot be produced by stops 

 within the draw tube. This objection applies to all methods 

 involving stops placed above the object itself, whether in the 

 draw tube or above the ocular as in the method described 

 below. 



(7) In the properly focussed microscope system the eye circle 

 is the conjugate image of the lower focal plane of the conden- 

 ser (fig. 7). A stop in the lower focal plane of the condenser 

 is accordingly imaged in the eye circle of the ocular, and vice 

 versa. In view of this mutual relation one might well conclude 

 that it is immaterial whether a stop be used in the eye circle or 

 in the lower focal plane of the condenser for producing oblique 

 illumination, but actual practice proves this to be not strictly 

 true, partly for the reasons cited under (6) above and partly 

 because of the imperfections in the lens system which is not 

 corrected primarily for the eye circle ; the result being that, 

 because of cumulative errors, an object in the lower focal plane 

 of the condenser is imperfectly imaged in the eye circle, and 

 the difficulty of properly placing a stop there is greatly 

 increased. This method, however, has often been suggested* 

 for producing oblique illumination, but it has not been actu- 

 ally used to any great extent by petrographers. It is inferior, 

 both theoretically and practically, to methods (2) and (3) 

 described above. 



Recapitulation. — In the above paragraphs the different 

 feasible methods for obtaining oblique illumination with the 

 petrographic microscope are described and their relative merits 

 briefly considered. From the discussion it is evident that the 

 methods are not all equally good either from a theoretical or from 

 a practical standpoint. Theoretically the first three methods 

 are superior to the others, and methods (2) and (3) are also sim- 

 plest to apply in actual work. They should, therefore, be 

 adopted in preference to the others. 



*S. Exner, Archiv Mikroskop. Anat., xxv, 97, 1885; F. Becke. Sitzungs- 

 ber. Acad. Wien, I Abt., cii, 358-376, 1893 ; Min. petr. Mitt., xiii, 385-388, 

 1893 ; J. W. Gordon, see Spitta, Microscopy, p. 179, 1909 ; F. E. Wright, 

 Carnegie Inst. Washington, 158, 95, 1911 ; H. Schneiderhohn, Zeitschr. 

 Kryst., 1, 234, 1912. 



