WRIGHT: PETROGRAPHIC MICROSCOPE 469 



in the image of the object plane. To accomplish this a sliding 

 stop or iris diaphragm is introduced just beneath the sliding posi- 

 tive lens; by raising the objective the object is imaged in the 

 plane of the stop, which allows only light from the particular 

 section to pass. In the writer's microscope the correct position 

 of focus is determined by means of the Bertrand lens which 

 together with the ocular constitutes a weakly magnifying micro- 

 scope focussed on the plane of the stop. 



For most petrographic microscope work relatively low magnifi- 

 cations are used and the astigmatism introduced by the nicol 

 is not a serious factor either in parallel or convergent polarized 

 light. It would seem, therefore, that except in work of precision 

 the above changes are in a sense refinements which add to the 

 complexity of the instrument. 



The assertion of Becher that the telecentric system permits 

 the use of the oblique end type of nicol prism as analyzer is not 

 borne out either by theory or by practice, because the oblique 

 ends of such a prism rotate the plane of vibration of the emergent 

 rays and are under all conditions inferior to the square end type 

 such as the Glan-Thompson prism. 



3. The prism method for the observation of interference figures. 

 In 1906 the writer described 5 a simple method for the observa- 

 tion of interference figures by the Lasaulx method; the method 

 consisted essentially in reflecting, by means of a prism mounted 

 in a slider, the rays of light from the axis of the microscope out 

 to a path outside the tube of the microscope, thus obviating the 

 necessity of removing the eyepiece each time an interference 

 figure is observed by the Lasaulx method. Improvements in 

 this device have been made by replacing the two reflecting prisms 

 by a single doubly reflecting prism; in reducing the size of the 

 prism so that it cuts out a small part of the center of the field 

 and thus serves as an effective stop sufficiently small to include 

 only one section or grain of ordinary size; and in placing the 

 prism slider directly beneath the field lens of the ocular so that 

 the part of the field covered by the prism can be viewed directly 



5 Am. J. Sci. (Series 4), 22: 19. 1906. 



