Ch. II] WORKING DISTANCE WITH THE MICROSCOPE 39 



can be easily corrected by getting one of the objectives in exact focus 

 and then turning the others successively into place. If one notes 

 whether it is necessary to focus up, then it will be known that the ob- 

 jective projects too far down toward the object; if, on the other hand, 

 one must focus down, then the objective is too high up. To correct 

 this lack of parfocalization use the objective which projects farthest 

 toward the object as standard. Focus it sharply and then turn another 

 in position. Unscrew this slowly untU the image is also sharp. Now 

 wind a thread or string around the lower end of the objective screw 

 and then turn it in place and slowly screw it into the revolving nose- 

 piece until it is in focus. Proceed with all until the entire number 

 are in focus at the same level. With parfocal oculars and parfocal 

 objectives much time and annoyance is saved, for one can see the 

 specimen in turning from power to power, and it is only necessary to 

 make a small focusing adjustment to get the best image. 



§ 76. Working distance. — By this is meant the space between 

 the simple microscope and the object, or between the front lens of 

 the compound microscope and the object, when the microscope is 

 in focus. This working distance is always considerably less than 

 the equivalent focal length of the objective. For example, the front- 

 lens of a 4 mm. objective would not be 4 millimeters from the 

 object when the microscope is in focus, but considerably less than 

 that distance, viz. less than half a millimeter. If now a cover-glass 

 of half a miUimeter or more in thickness were used it would be impos- 

 sible to get the 4 mm. objective near enough the object to get it in 

 focus. It is not uncommon for students to put their microscopic 

 specimens on the stage of the microscope wrong side up. Then the 

 thickness of the sUde is over the object. With low powers the object 

 can still be put in focus; but not with high powers, as the working 

 distance is not great enough. See also aberrations produced by the 

 cover-glass (fig. 51). 



§ 77. Free working distance. — (i) Where no cover-glass is used 

 this is the distance between the front of the magnifier or the front 

 lens mount of the objective and the object (fig. 30). 



(2) If a cover-glass is used, it is the distance between the upper 

 surface of the cover-glass and the magnifier or objective when the 



