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TM-9-254 Gearcase Transfer M548 M548A1 (
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APPENDIX A REFERENCES - TM-9-254_311
TM 9-254 10-10. Lens Bench - Continued (9) Eccentricity.    A  lens  is  said  to  be  concentric  when  the  geometrical  or  mechanical  axis  of  the  lens coincides  with  optical  axis.    When  a  lens  is  concentric,  there  will  be  no  perceptible  movement  of  the image  when  the  lens  is  rotated  about  its  geometrical  axis.    Conversely,  when  these  two  axes  do  not coincide,  the  lens  is  eccentric,  and  the  amount  of  eccentricity  is  proportional  to  the  movement  of  the image  when  it  is  rotated  about  its  geometrical  axis.    When  the  optical  bench  is  used  to  measure  the eccentricity  of  a  lens,  the  regular  chuck  in  the  nodal  slide  is  replaced  by  a  special  chuck,  which  will permit the lens to be rotated freely on an axis parallel to the ways of the bench and coincidental with the axes of the collimator and microscope.  When a lens is mounted in the chuck and is rotated through 360 degrees, the image of the intersection of the cross lines will trace a circle against the target, as seen in the  observing  microscope.    The  radius  of  this  circle  is  directly  proportional  to  the  eccentricity  of  the element   under   observation.      A   convenient   target   for   measuring   the   actual   amount   of   eccentricity consists of a group of twelve concentric circles whose centers are at the center of the target and whose radii  will  indicate  a  series  of  deviations  from  1  to  12  minutes  for  the  particular  focal  length  of  the collimator being used. 10-13/(10-14 blank)

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