468 WRIGHT: PETROGRAPHIC MICROSCOPE 



Some of the most important measurements in petrologic micro- 

 scopic work are made in the interference figure (optic axial angles 

 and angular distances of isochromatic curves), and it is highly 

 desirable that a method be available for eliminating the astig- 

 matism and distortion from the interference figure, 4 as this 

 seriously affects the degree of attainable precision. The inter- 

 ference figure is formed in the upper focal plane of the objective 

 and the rays from its points are imaged, after passage through 

 the analyzer and Bertrand lens, in the image plane of the eye- 

 piece. In order to fulfil the condition that these rays enter the 

 nicol as parallel beams, a weak positive lens must be introduced 

 below the analyzer in such a position that the lower focal plane 

 of the lens coincides with the plane of the interference figure. 

 The rays after emergence from the analyzer are focussed by 

 means of a weak positive lens in the image plane of the eyepiece. 

 In effect this arrangement is that of two astronomical telescopes 

 in series, the objective and lens below the nicol forming the first 

 system, the lens above the polarizer and the eyepiece the second. 



The practical problem, to .combine the arrangement for elimi- 

 nating astigmatism in parallel polarized light with that for con- 

 vergent polarized light, is being met on the writer's microscope 

 as follows: The negative lens and the positive lens required for 

 the correction in parallel polarized light are fitted below and 

 above the nicol respectively. To pass to convergent polarized 

 light a positive lens is introduced in a slider in a slot between 

 the analyzer and the objective; the focal length of this lens is 

 such that its combination with the negative lens functions as a 

 positive lens whose lower focal plane coincides with the plane of 

 the interference figure. By this arrangement the interference 

 figure is imaged slightly magnified (magnification approximately 

 equal to the ratio of the focal length of upper positive lens to that 

 of combination positive and negative lens below the analyzer) 

 in the image plane of the eyepiece. In the case of small sections 

 it is essential that all extraneous light from adjacent sections be 

 excluded from the field ; this can only be done by means of a stop 



4 See, F. E. Wright. Methods of petrographic microscope research. Carnegie 

 Institution of Washington, Publication No. 157, pp. 53-56. 1911. 



