54 



LIGHTING AND FOCUSING. 



[CH. II. 



\ Object-b 



Object-a 



Fig. 57. — Figure to show that in lengthening the tube of the microscope the ob- 

 ject must be brought nearer the principal focus or center of the lens. It will be seen 

 by consulting the figure that in shortening the tube of the microscope the object 

 must be removed farther from the center of the lens. By consulting the figure 

 showing the effect of the cover-glass (Fig. 36) it will be seen that the effect of the 

 cover-glass is to bring the object nearer the objective, and the thicker the cover the 

 nearer is the object brought to the objective. As shortening the tube serves to re- 

 move the object, it neutralizes the effect of the thick cover, and if the cover is so 

 thin that it does not elevate the object enough for the corrections of the objective, 

 then an increase in the tube-length will correct the defect. 



Furthermore, whatever the interpretation by different opticians of 

 what should be included in " tube-length," and the exact length in mil- 

 limeters, its importance is very great ; for each objective gives the most 

 perfect image of which it is capable with the ' ' tube-length ' ' for which 

 it is corrected, and the more perfect the objective the greater the ill- 

 effects on the image of varying the " tube-length " from this standard. 

 The plan of designating exactly what is meant by " tube-length," and 



