,500 



Dr. G. J. Stoney on Microscopic Vinton. 



The X scale may be conceived as lying along the line bp, 

 its zero being at o, and the number n (the index of refraction 



of the air or oil that is in front of the objective) being at the 

 point p on the scale. 



The image plane means the plane in which image C lies. 

 (See p. 504.) _ 



33. Numerical Aperture, or Grasp. — The microscopic 

 object, the illumination supplied to it, the cover-glass placed 

 over it, and the media on both sides of this cover-glass — viz. : 

 the medium between the cover-glass and the object on the 

 one side, and the medium between the cover-glass and the 

 objective on the other (which latter is usually air or oil) — are 

 what between them determine the condition in which light 

 enters the objective. 



It will be convenient to call the three media between the 

 object and the objective a,b, c; a being the medium in which 

 the object is mounted, b the glass of which the cover-glass is 

 made, and c being the oil or air which intervenes between the 

 cover-glass and the objective. The surfaces of demarcation 

 between these media are always planes perpendicular to the 

 axis of the microscope, so that we need only consider this 

 simple case. 



It has already been proved that the light immediately in 

 front of the object, i. e., as it exists while in medium a, may be 

 resolved into beams of uniform plane waves. These beams con- 

 tinue to be beams of uniform plane waves in travelling across 

 media b and c, since the surfaces separating the three media 

 are planes. Hence the light immediately before entering the 

 objective, i. e., as it exists in medium c, may be resolved into 

 beams of uniform plane waves. We may regard this light as 

 suffering reversal, as in § 8, and at the same time conceive 

 medium c, the oil or air in front of the objective, to be 

 extended downwards, all other apparatus beneath * being 

 removed. 



* For convenience of description we suppose the microscope to be 

 pointed downwards, as it usually is more or less. It should be noted 

 that what is commonly called the front of the object is what is then its 

 upper side, the side turned towards the observer, while what is called 

 the front of the objective is its under end, that end which receives the 

 lisrht. 



