USING THE MICROSCOPE. 11 



use. In order to focus correctly, we first slide the tube down- 

 wards (carefully controlling its movement by having one finger 

 partly on the sliding-tube and partly on its sheath) until it almost 

 touches the object. Then, while looking through the eye-piece, 

 we move the tube as slowly as possible upwards, controlling its 

 movement in the same way. This sliding movement is best 

 combined with a slight zigzag twisting (not rotation) of the' tube 

 inside the body sheath. Soon the position is reached in which 

 the previously invisible object begins to show in the form of 

 small grains. If, however, we find that we have withdrawn the 

 objective to more than about ^ of an inch from the object -slide, 

 without having caught sight of the grains, these either do not lie 

 in the field of view of the microscope, or else we have raised the 

 tube too quickly, so that the rapidly appearing and equally rapidly 

 disappearing object has escaped our observation. If that be so, 

 we must not attempt to find the object by sliding the tube 

 downwards, as thereby we should run into the danger of breaking 

 the cover-glass, injuring the object, and dirtying or even injuring 

 the objective ; instead of this we again slide the carefully 

 controlled tube so far downwards that it almost touches the 

 object-slide, and begin anew to raise the tube more slowly than 

 before, at the same time looking through the eye-piece. If you 

 do not happen to see the object itself, the focus may often be 

 approximately found by means of specks upon the cover-glass. 

 If we do not succeed in this second attempt we may assume that 

 the object does not lie in the field of view, which after all is a 

 very limited one ; and we must look for it again after altering the 

 position of the object-slide. After some attempts the grains at 

 length appear in the field of view, and we then discontinue sliding 

 the tube, that is, using what we call the coarse adjustment, and 

 effect the fine focussing or adjustment by means of the micrometer 

 screw (Fig. 3, M). This we turn in either direction, but in case 

 the object is made thereby more indistinct instead of clearer, we 

 move it in the opposite direction. This adjustment, or focussing, 

 is perfect when the figure appears as sharp as possible. In our 

 example of a microscope stand (Fig. 3), the micrometer screw is 

 at the upper end of the pillar ( p ) ; but it can be variously 

 placed according to the make of the instrument, though this is 

 its usual position. If the stand we have in use possesses rack 

 and pinion for coarse adjustment, it must be used with great care, 

 since it is easy to overwind the screw, and cause a much more 



