CONSTRUCTION OF THE MICKOSCOPE. 41 



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 optic 

 axis of the instrument should not be in the least altered by any move- 

 ment in a vertical direction; so that if an object be brought into the 

 centre of the field with a low power, and a high power be then substi- 

 tuted, the object should be found in the centre of its field, notwithstand- 

 ing the great alteration in the focus. In this way much time may often 

 be saved by employing a low power as a 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, 23), a rack-and '-pinion adjustment, if it be 

 made to work both tightly and smoothly, 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/ or ' slow motion/ 

 by means of a screw -movement operating either on the object-glass alone 

 or on the entire body, is of great value; and for the highest 1 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 only admissible 

 where, for the sake of extreme cheapness or portability, the instrument 

 lias to be reduced to the form of utmost simplicity. 



in. Scarcely less important than the preceding requisite, in the case 

 of the Compound Microscope, especially with the long body of the 

 ordinary English model, is the capability of being placed in either a 

 vertical or a horizontal position, or at any angle with the horizon, without 

 deranging the adjustment of its parts to each other, and without placing 

 the eye-piece in such a position as to be inconvenient to the observer. 

 It is certainly a matter of surprise, that some Microscopists, especially on 

 the Continent, should still forego the advantages of the inclined position, 

 these being attainable by a very small addition to the cost of the instru- 

 ment; but the inconvenience of the vertical arrangement is much less 

 when the body of the microscope is short, as in the ordinary Continental 

 model; and there are many cases in which it is absolutely necessary that 

 the stage should be horizontal. This position, however, can at any time 

 be given to the stage of the inclining Microscope, by bringing the" optic 

 axis of the instrument into the vertical direction. And even with the 

 stage horizontal, a convenient inclination may be given to the visual axis, 

 not merely by such modifications in general construction as constitute 

 the special features of the erecting Binocular of Mr. Stephenson ( 36) 

 or the Inverted Microscope of Dr. Lawrence Smith ( 80), but by the 

 application to the ordinary vertical body of the erecting eye-piece of M. 

 Natchet ( 8G). In ordinary cases an inclination of the body at the angle- 

 of about 55 to the horizon will usually be found most convenient for 

 unconstrained observation; and the instrument should be so constructed, 

 as, when thus inclined, to give to the stage such an elevation above the 

 table, that, when the hauds are employed at it, the arms may rest con- 

 veniently upon the table. In this manner a degree of support is attained, 

 which gives such free play to the muscles of the hands, that movements- 

 of the greatest nicety may be executed by them; and the fatigue of long- 

 continued observation is greatly diminished. Such minutiae may appear 



