COARSE AND FINE ADJUSTMENTS. 



into an expedient, by which the inequalities of the surface of a 

 microscopic object are gauged, and its accidents analysed. Thus, 

 for example, let the milled head of the fine adjustment be first 

 turned so as to render the salient parts distinct, and let the 

 position of the index be marked. Let it be then turned so as to 

 render the depressed parts distinct, and let the new position of 

 the index be marked. If one division of the head represent the 

 1500th part of an inch, the differences of level, of the salient 

 and oppressed parts, will be just so many loOOths of an inch as 

 there are divisions of the milled head which have passed the 

 index. 



36. One of the first difficulties which the microscopic debutant 

 encounters, is that which will attend his attempts to bring the 

 object into the centre of the field of view when it is minute, and 

 when the magnifying power is considerable. If he is only pro- 

 vided with a simple stage, without any mechanical expedient for 

 moving the object, he will soon be oppressed with the fatigue 

 arising from a succession of abortive attempts at accomplishing 

 his purpose. 



37. The entire diameter of the field of view will often be less 

 than the 100th of an inch, so that a displacement of the slide so 

 inconsiderable as to be utterly insensible to his fingers, will cause 

 the object to jerk through a space greatly exceeding the entire 

 extent of the field. In this way the object will start from side 

 to side, the motion imparted to it by the touch to bring it back 

 to the field being always in excess, however carefully and deli- 

 cately the manipulation may be made. Some professional 

 observers, by intense and long-continued practice, surmount this 

 difficulty and succeed in adjusting the slides, even with the 

 highest powers, without mechanical aid ; but this is not to be 

 hoped for by debutants or amateurs, except with very low magni- 

 fying powers. Such persons, if they would avoid the risk of 

 throwing up the instrument with disgust, had therefore better in 

 all cases be provided with a stage having some such expedients as 

 we shall now describe. 



Upon the fixed stage, such as it has been described, a second 

 stage similar in form and equal in size is placed, and is moveable 

 through a certain limited space right and left, by a fine screw 

 with a milled head. Another similar stage is placed upon this, 

 which partakes of any motion imparted to the latter, but which 

 is also moveable upon the latter backwards and forwards by means 

 of another fine screw. Upon this last stage the slide with the 

 object is placed, and held down by springs so as to retain its 

 place, whatever be the position of the stage. 



By turning one of these screws (fig. 19), the object may be 



37 



