1020 Prof. Wood and M. L. Dunoyer on Separate Excitation 



a block of quartz of very large size must be used, placed 

 between the two halves of the condenser, since the rays which 

 traverse the block must be parallel. If this is not the case, 

 different pencils will traverse different thicknesses, and will 

 be differently polarized. Moreover, one half of the light is 

 lost at the start by the polarizing nicol. This difficulty was 

 overcome by employing a large double-image prism, and 

 subsequently analysing by a double-image prism. In this 

 way, with proper orientation of the prisms, the two images 

 containing only D 2 light were superposed, the D x images (of 

 one half the intensity) lying to the right and left. By this 

 expedient the D 2 image had the full intensity, except for the 

 loss by reflexion from the six transparent surfaces of the 

 prisms and quartz block. 



The quartz block used in the experiment was prepared by 

 M. Bertin from a selected crystal of Madagascar quartz, 

 which was previously examined by sodium light between 

 crossed nicols to make sure that no irregularities of crystal- 

 lization were present. The block measured 85 mm. X 60 mm. 

 X 32 mm. and gave excellent results. 



The arrangement of the apparatus is shown in the diagram. 



Ffc. 1. 



Oooh/e leasee 



Blast ^ CMUer 



The double-image prism A is so oriented as to have the 

 vibrations of the two transmitted beams at 45° with the 

 vertical. The second double-image prism is oriented in the 

 absence of the quartz block, so as to give two images of the 

 source side by side, and separated by a distance equal to the 

 width of each image. Each image contains both I^ and D 2 . 

 If now the quartz block is placed between the two halves of the 

 Dunoyer condenser, all of the D 2 light (for example) leaves 

 the original images, and unites into a third image between 

 the other two. This is the image utilized. If it is desired 

 to have J) 1 light in this image, it is necessary only to rotate 

 the block a degree or two, thereby increasing the optical 

 path in quartz. If we rotate the analysing double-image 

 prism the central image splits up into two, and the correct 





