MECHANICAL REQUIREMENTS. 87 



the adjustment of the illuminating apparatus must he made 

 afresh for every change of magnifying power; whilst if the 

 stage he a fixture, the illumination having heen*once well 

 adjusted, the object may be examined under a great variety of 

 magnifying powers, without its being changed in any respect. 

 Moreover, if the stage be the movable part, it can never have 

 that firmness given to it which it ought to possess; for it is 

 almost impossible to make a movable stage free from some 

 degree of spring, so that, when the hands bear upon it in adjust- 

 ing the position of an object, it yields in a degree, which, how- 

 ever trifling in itself, becomes unpleasantly apparent with high 

 powers. The mode of efiecting the focal adjustment should be 

 such as to allow free range from a minute fraction of an inch to 

 three or four inches, with equal power of obtaining a delicate 

 adjustment at any part. It should also be so accurate, that the 

 optical axis of the instrument should not be in the least altered 

 by movement in a vertical direction; so that, if an object be 

 brought into the centre of the field with a low power, and a 

 higher power be then substituted, it should be found in the centre 

 of iU field, notwithstanding the great alteration in the focus. In 

 this way much time may often be saved, by employing a low 

 power as & finder for an object to be examined by a higher one ; 

 and when an object is being viewed by a succession of powers, 

 little or no readjustment of its place on the stage should be 

 required. For the Simple Microscope, in which it is seldom 

 advantageous to use lenses of shorter focus than l-4th inch (save 

 where doublets are employed, § 18), a rack-and-pinion adjustment 

 answers sufficiently well ; and this is quite adequate, also, for the 

 focal adjustment of the Compound body, when objectives of low 

 power only are employed. But for any lenses whose focus is less 

 than half an inch, a "fine adjustment," by means of a screw move- 

 ment operating either on the object-glass alone or on the entire 

 body, is of great value ; and for the highest powers it is quite 

 indispensable. In some Microscopes, indeed, which are provided 

 with a "fine adjustment," the rack-and-pinion movement is 

 dispensed with : the "coarse adjustment" being given by merely 

 sliding the body up and down in the socket which grasps it. But 

 this plan is objectionable, inasmuch as it involves the use of both 

 hands in making, the "coarse adjustment," for which only owe 

 should be required ; and even then the adjustment cannot be made 

 with nearly the same facility, as by a smooth well-cut rack. The 

 Author's experience, therefore, would lead him to recommend, 

 that if one of these adjustments is to be dispensed with, it should 

 be the "screw" or "fine" adjustment, rather than the "rack" 

 or "coarse," unless the instrument is to be almost exclusively 

 employed for the examination of objects requiring high magnify- 

 ing powers.' 



' In the Microscopes constructed by Mr. Ladd, a chain-movement is substituted for the 

 rack-and-pinion; and this has the advantage of being smoother and more sensitive, of 

 being less lilcely to become unequal by wear, and of being easily tightened if it should 



