708 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



microscopic image from left to right. The reading on the ocular micro- 

 meter now gives the depth. The slit-cheeks are brought as closely 

 together as is suitable for the purpose. 



It remains now to delimit forwards and backwards a portion of the 

 light cone ; and this is done by rotation of the ocular micrometer into 

 the position shown in fig. 113.. The gauging can also be done by 



JLA. 



(.4; :i.+'>Vf ft {'I' "I! 

 30 kU .50 



Fig. 113. 



placing in the ocular quadratic fields, whose dimensions are known. 

 Fig. 114 shows the network in Huyghens' ocular 4, which contains 

 eighteen squares. The side-length of such a square has, in the com- 

 1 bination of water immersion D* with a tube-length of 1G0 mm., a value 

 of about /a shown on the object. This arrangement suffices for 

 approximate measurements ; but for accurate determinations the ob- 



Fig, 114. 



server must ascertain its value for his objective and ocular by com- 

 parison with an object micrometer. 



Finally, for the observation of polarisation effects, an analyser is set 

 up on the ocular. The particles show themselves the more polarised in 

 proportion as they are more minute, according to the plane which passes 

 through the axis of the illuminating and refracted beams. The analyser 

 also serves for distinguishing the non-polarised fluorescent light from 

 the refracted light. 



