OIL-IMMERSION OBJECTIVES 69 



later on, in reference to the part of the diagram below 0. 

 In the figure the mounting medium, the glasses, and the 

 immersion fluid are all represented as of the same re- 

 fractive index 1-515, that of cedar oil, and consequently 

 the ray OK passes through the system without any 

 refraction. AOK=a is half the angle of aperture of the 

 objective whose N.A. (oil) =n sin a, where n = 1-515. 



Now suppose the objective is used dry, but the object 

 still mounted in the same medium as shown on the right- 

 hand side of Fig. 21. A ray leaving at the angle a to 

 the perpendicular no longer enters the objective, but is 

 refracted at the surface of the cover-glass to N. Suppose 

 c^ is the greatest angle at which a ray can leave O and 

 still be received by the objective. It will be refracted at 

 L, forming there an angle a to the perpendicular, and 

 the numerical aperture of the dry lens is given by 



=1 sn,, 



(N.A.) " n sin a n 



That is, the N.A. of the objective is increased n times 

 when immersion oil is used. 



It follows that much higher resolution can be achieved 

 by an immersion over a dry lens of the same magnifica- 

 tion, whilst at the same time the image is much brighter. 

 For this reason the lenses of higher power than J" are 

 now almost universally made for oil immersion, and have 

 a large N.A. of 1-2 to 1-4. Their increased resolving 

 power over dry lenses of similar magnification is very 

 marked, and the exposures when they are used for 

 photomicrography may even be less than those required 

 by dry lenses of half their power. 



Correction for Cover-Glass Thickness. Owing to refrac- 

 tion in the cover-glass the optician has to compute his 

 objectives for a standard thickness of cover-glass, giving 

 a definite amount of refraction. 0'18 mm. is that usually 

 chosen and marked on the objective. For low powers 



