W. N. Benson — Model for a Polarizing Microscope. 447 



VI. — A Model for a Polarizing Microscope. 

 By W. N. Benson, B.A., B.Sc, Emmanuel College, Cambridge. 



PROBABLY most teachers of petrology have experienced difficulty 

 in making clear to some elementary students the manner in 

 which light is transmitted through a polarizing microscope, and the 

 explanation of birefringence and pleochroism. In such circumstances 

 I have found very helpful the analogies illustrated by an easily 

 constructed model. Doubtless such an instrument has been used 

 before, but, as the textbooks make no mention of it, a description 

 may be of use here. 



It consists of a number of portions strung on a stout wire or 

 knitting-needle {Hit in figure), which represents the path of the 

 pencil of rays along the axis of the microscope. The rays falling on 

 the mirror A (a cork) are reflected along H R, and the pencil is made 

 up of rays vibrating across EH in every azimuth (as shown by i?). 



BB, path of ray through microscope axis (lenses not represented) ; the arrows 

 indicate the planes of vibration; A, sub-stage mirror; C, polarizer; 

 D, plane polarized light ; F, crystal plate ; H, cross-wires ; J, analyser ; 

 Ko Ke, interference of vibrations brought into the same plane by the 

 analyser (birefringence) . 



On reaching C, the polarizer, each of these rays is resolved into two 

 directions, but is transmitted along one only of these directions 

 (through the slot C). Thus all the light leaving the polarizer 

 vibrates in one plane, as shown by the arrow through the cork D, 

 which is rigidly connected with C. F, representing the crystal 

 plate, rotates freely about JRH. The light from the polarizer is 

 resolved into two directions perpendicular to each other, and is trans- 

 mitted through the crystal as through the slots o and e. In this 

 passage, however, one ray is retarded more than the other, so that 

 the nature of the light as it leaves the crystal plate is illustrated by 

 the two arrows o' and e' driven through the cork G, which is rigidly 

 connected to F. As the crystal is rotated the intensity of the o and 

 e rays vary, and this may be indicated by pushing the corresponding 

 arrows in or out of the cork G. The board 5" shows the position of 

 the cross-wires in the microscope, and serves to hold R R in place. 

 The analyser J, and vibration arrows Ilo and JTe are attached to the 

 cork i, which slides on or ofE RR. When properly in position for 



