A SIMPLE DESCEIPTION OF THE MICROSCOPE 17 



The 1/6-incli is receiving from the object, cones of light Angle in air 

 of 116°, as shown in Fig. 6. It could not be made to coUect Z^hZlm 

 a very much larger angle of light because it cannot be used in s^^- 

 absolute contact with the object. Sufficient space must be 

 provided for a thin glass cover and a small distance for focussing 

 adjustment. It will be noticed in Fig. 6 that the cone of light, 

 which is 116° as it enters the 



object glass, is only 68° when it ^i^ 



passes through the object. It M^ 



is spread out by refraction as it i^^ 



enters the air between the cover /^^^^ ^-^10.51^ 



glass and the lens. If the air ^^^^^^-' ,j^,f4«, 



space between the cover glass • ;»u;;;:!^j:!-Ws j' UiC-^:?'^-M^-':^:^ ly, 



and the lens could be filled up "i^ii^^i^^M^^i^y^^::'-^--^' ^ 

 with glass, this spreading out of 



c. - 



the cone would not occur, and ' Jl5°' 



the cone of light would remain Fig. 6. 



68° when it entered the lens. 



As far as the power of depicting detail is concerned it would be 



equal to a 116° cone in air. It is the same body of light and has 



just the same properties in this respect. If therefore the space 



between the object and the lens is glass throughout, a larger 



angled cone than 68° can be collected by the object glass, and a 



greater power of depicting detail, what is known as resolution, 



can be reached, and a further power of seeing fine structure 



obtained. 



Cedar-wood oil is a liquid which has the optical properties of immersion 

 glass, and if a drop of this oil is placed between the front of °^l^^ 

 the object glass and the cover glass, the whole distance between 

 the object and the lens is equivalent to glass. A much larger 

 effective aperture can thus be obtained with corresponding 

 increase in resolution. 



Thus object glasses of higher power than 1/6 inch (4 mm.) 

 are generally what are called immersion object glasses. They are 

 so constructed that a drop of cedar-wood oil must be placed 

 on the front lens so that it connects it to the object being 

 examined. 



The method of describing the aperture by the term numerical Numerical 

 aperture (N.A.) instead of by the actual angle of the cone is to ^p*"^'""- 

 enable the resolving power of a microscope to be correctly stated. 

 A 1/12-inch oil-immersion object glass is generally made to admit 

 an angle in glass of 117°, which corresponds to an angle of more 

 than 180° in air. It is almost the same actual angle as the 1/6-inch 

 admits from air, but the numerical aperture (N.A.) which gives 

 its true resolving power is I'S N.A., while that of the 1/6-inch is 

 only -85 N.A. 



Dry lenses such as the 1/6-inch cannot be used with cedar- 

 wood oil as immersion lenses, and immersion lenses cannot 



