22 



THE MICROSCOPE 



Vision ot 



natural 



objects. 



The mirror. 



Direction of 

 the light. 



The same holds true of opaque objects examined with the 

 microscope, but the greater mimber of microscope specimens are 

 either transparent or semi-transparent, and must be viewed by 

 sending a beam of light through them from behind. This beam 

 then passes through the microscope into the eye. Natural objects 

 are seldom viewed in this manner, but in order to examine the 

 water mark of paper or a photographic transparency, they must 

 be held between a strong light and the eye, and the ability to see 

 the pattern of the water mark or the view in the transparency 

 depends on certain portions of the light being blocked out which 

 would otherwise enter the eye. In the black portion the whole 

 light is stopped, in others only a portion is absorbed, and thus a 

 complete range of tone in the picture may be obtained. This is 

 the method by which semi-transparent objects are seen with the 

 microscope. 



The conditions are not the same as ordinary vision, and the 

 direction and character of the beam of light used to illuminate 

 them are a matter of great importance. 



The mirror of the microscope (F, Fig. 1) is used to direct a 

 beam of light from some source of illumination through the 

 object into the microscope. The mirror swings in gimbals and 

 can be moved in all directions. It has 

 on one side a flat, silvered surface 

 which gives a plane reflection, and on 

 the other a concave surface which 

 concentrates a more powerful beam 

 upon a small area of the object. 



The direction of the light should be 

 such that it shines directly along the 

 line that passes through the centre of 

 the microscope — the line that is known 

 as the optic axis. If the light comes 

 from the side it passes obliquely 

 through the object, and even if it does 

 not give an erroneous appearance it 

 prevents a clear image being formed. 

 To illustrate the efiect on the object, 

 one should examine the appearance of 

 a fairly thick piece of wood which has 

 fine holes drilled in it : light from one 

 side would not pass through and would 

 not reach the eye. The holes will not 

 be visible unless the light is passing 

 through them centrally. The effect of 

 an oblique beam of light as it passes 

 through the microscope lenses is shown in Fig. 10. 



The left-hand diagram in Fig. 10 shows the object glass 

 transmitting oblique light only. The light which actually forms 



Oblique light. Direct light, 

 Fig. 10. 



