42 WORKING DISTANCE WITH THE MICROSCOPE [Ch. II 



justment head at zero (o). Lower the objective carefully with the 

 coarse adjustment until the objective just touches the cover-glass. 

 Now focus up with the fine adjustment until the object is in sharp 

 focus, noting the total and partial revolutions of the screw to accom- 

 pUsh this. The distance the objective was raised is the free space 

 between the front of the objective and the cover-glass. Suppose it 

 required 3.2 revolutions of the fine adjustment to focus the objective, 

 then if each revolution represents o.i mm. the total elevation is 3.2 X 

 0.1= 0.32 mm. for the free working distance in this case. 



§ 80. Effect of the cover-glass on the working distance. — It is 

 obvious that if an object is covered with a layer of glass that the free 

 space between the front of the objective and the object will be lessened, 

 and if the layer of glass is considerably thicker than the working dis- 

 tance of the objective, then it will be impossible to get the object in 

 focus. If the layer of glass is relatively thin, then it will be possible 

 to focus the microscope on the object, but from the law of refraction 

 it necessarily follows that the focus of the microscope with and without 

 a cover-glass wiU not be the same. 



Now from the refraction of the rays in passing from one medium to 

 another of different refractive power, it follows that, when an object 

 is in or below a stratum of glass or water or other highly refractive 

 body, the object will appear as if raised (fig. 31, 51), the amount of 

 the apparent elevation depending on the refractive index of the cover- 

 ing body, — the greater its refraction, the more the apparent elevation. 

 The general physical law is that the eye being in the air the apparent 

 depth of an object below the surface when viewed perpendicularly is 

 the actual depth multiplied by the reciprocal of the index of refraction 

 of the covering body. The index of refraction of the cover-glass is 

 1.52 or approximately 1.50, and its reciprocal is ji^ = f . That is, 

 the apparent depth is only | its actual depth, or in other words the 

 object seems to be elevated ^ of the actual depth. 



Now if the object is apparently higher up, the microscope must 

 be raised an amount equal to the apparent elevation of the object. 

 This is illustrated in fig. 31-32. From this it follows that the free 

 working distance of the objective on a covered object is not lessened 

 the full thickness of the cover-glass, but only f of that thickness. 



