616 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



A, Instruments, Accessories, etc.* 

 (1) Stands. 



Model for a Polarizing Microscope.f — W. N. Benson has designed 

 this model for the purpose of demonstrating to students the manner in 

 which light is transmitted through a polarizing Microscope, and the 

 explanation of birefringence and pleochroism. 



The model consists of a number of portions strung on a stout wire or 

 knitting-needle (ER in fig. 80), 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 RR, and the pencil is made up of rays vibrat- 

 ing across RR in every azimuth (as shown by B). On reaching C, the 



Fig. 80. — RR, path of ray through Microscope axis (lenses not re- 

 presented) : 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 vibra- 

 tions brought into the same by the analyser (birefringence). 



polarizer, each of these rays is resolved into two directions, but is trans- 

 mitted along one only of these directions (through the slot C). Thus 

 all the hght leaving the polarizer vibrates in one plane, as shown l)v the 

 arrow through the cork D, which is rigidly counected with C. F, repre- 

 senting the crystal plate, rotates freely about RR. The light from the 

 polarizer is resolved into two directions perpendicular to each other, and 

 IS transmitted 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 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illummatmg and other Apparatus; (4) Photomicrography; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



t Geological Mag., x. No. 10 (1913) pp. 447-8 (1 fig.). 



