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in the centre of the field, produces again white light. Any object, 

 therefore, placed in the white field, partakes of the characteristics of 

 the selenite; one image being refracted into the plane which exhibits 

 the green ray, assumes that colour, whilst the second image being re- 

 fracted into the plane of the red ray, partakes of that tint. Should, 

 however, the object be so large that actual separation of the images is 

 not effected, the extremities and interstices only will be polarized, 

 whilst the middle will remain dark, or present only its natural hue. 



