THE OPTICAL EQUIPMENT 



37 



be effected when the objective-changers are attached to the 

 lenses with which they are to be used. Having once been 

 properly centred and adjusted, they should not be taken off 

 their respective objectives, but should always be maintained 

 in position. The objectives will then always return to their 

 proper position exactly in line with the optical axis of the 

 instrument. These appliances are so arranged" that when 



FIG. 10. Sliding Objective-changer. (Zeiss.) 

 a, tube-slide ; &, objective-slide with objective attached ; c, intersection. (Full size.) 



sliding out a lens the direction of motion is upwards, and 

 therefore away from the object on the stage ; there is 

 therefore little likelihood of damaging the object on the 

 microscope stage if the objective is changed, without racking 

 up the body-tube. 



It should be carefully noted when using either the nose-piece 

 or objective-changers that due allowance must be made for 

 these in estimating the tube-length. (See Chap. VII, p. 151.) 



A simple objective-changer which is not open to the objections 

 urged against those of the double or triple nose-piece type is 



