THE MICROSCOPE. 



part to be moved is attached. By turning this milled head one 

 way and the other alternately, the observer finds by trial the 

 position which gives greatest distinctness. 



33. This, which is called the COARSE ADJUSTMENT, answers 

 well enough when high powers are not used ; but it must be 

 remembered that as the teeth of the pinion successively pass those 

 of the rack, the motion produced is not strictly an even and 

 uniform one, but a sort of starting or intermitting motion, so that 

 the instrument cannot be easily and steadily brought to rest at 

 any intermediate point between the beginning and the end of the 

 passage of a tooth. When high powers are used, and conse- 

 quently an extremely nice adjustment of the focus required, this 

 arrangement is therefore insufficient, and serves at best only for a 

 first approximation to the exact focus. 



34. A supplemental expedient is therefore provided in the 

 best instruments, called the FIXE ADJUSTMENT, which usually 

 consists of a screw having an extremely fine thread, which 

 being connected with the part to be moved, gives it a per- 

 fectly smooth, uniform, and slow motion, entirely free from starts 

 or jerks. 



In some of the best instruments these screws have as many as 

 150 threads to the inch, so that one complete turn of the milled 

 head moves the stage or body through only the 150th part of an 

 inch, and as the head is divided into ten equal parts and moves 

 under an index, a tenth of a revolution can be observed, which 

 corresponds to the 1500th part of an inch. 



When the form of the object is not actually flat, and conse- 

 quently all points upon it are not equally distant from the object- 

 glass, they will not be all in focus together. When the distance 

 of the object is such as to bring the more salient, and conse- 

 quently the nearest, parts into focus, the more depressed parts 

 will be too distant, and consequently out of focus ; and when the 

 object is moved nearer to the object-glass by a space equal to the 

 heights of the salient above the depressed parts, the latter will be 

 in, and the former out of focus, and consequently the latter will 

 be distinct, and the former confused. 



When the powers used are so low that the distance of the 

 object from the object-piece shall bear a considerable proportion 

 to the difference of level of the salient and depressed parts of the 

 object, this difference of level will not sensibly affect the focal 

 adjustment ; but when high powers are used, that difference of 

 level bearing a very sensible proportion to the distance of the 

 object from the object-glass, the adjustment which renders either 

 distinct will render the other indistinct. 



35. This optical fact has been converted with admirable address 



