166 



PROGRESS IN MICROSCOPY 



Let dp be the depth of accommodation in metres, G the microscope 

 magnifying power, 250 millimetres the reading distance. 



If .T is the accommodation amplitude in dioptrics, u the immersion 

 index, then: 



dp 



|0-25i^ 

 n\~—- rt 



(5.4) 



Let P be the distance to the near point and R to the far point. These 

 two distances, showing in metres the accommodation amplitude (in 

 dioptrics), is educed by: 



1 1 



(5.5) 





R 



If 0) is the exit pupil diameter, the expression may be written thus: 



dp = 



Tior 



Ansm^u 



(5.6) 



Hence, if the accommodation is to be reduced as far as is feasible, 

 the exit pupil must be as small as possible, i.e. magnify to the utmost, 

 for a given objective. 



It may be required to ascertain the admissible eye-accommodation 

 variation enabling one to obtain the accuracy shown theoretically 

 by diffraction (equation 5.2): Let n denote such variation. The 

 resulting data are tabulated hereunder: 



Regardless of eye accommodation (the accommodation amplitude of 

 a normal eye is 10 dioptrics), a 0-2 mm exit pupil provides theoretical 

 accuracy. Therefore, such an exit pupil does not require a reticle. 

 It should be pointed out that such small exit pupils are not advocated 

 (short focal-length eyepieces entail intricate design and prominent 

 entoptic phenomena). If now, the eyepiece is provided with a reticle, 

 it is assumed that a trained eye can be accommodated steadily with 

 01 dioptric. Under such conditions, with an eyepiece comprising 

 a reticle and in accordance with the foregoing table, exit pupils ex- 



