PRACTICAL EXERCISES 



1107 





Fig. 479- 



Fig- 478J i r>lane being moved to 



eve, the other being 

 Led, and move over 

 a pencil 



c 

 the 



paper, 'until ^- ~ _ 

 fust disappears. Make 

 a mark on the pape 

 atthispoint.andrepeat 

 ?he observation for all 

 diameters of the field. 

 ?hf blind spot is thus 

 marked -^ Ficr - A8 ^' 



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