MICROSCOPE. 



345 



so that it may be free from vibration. An 

 amount of vibration, which is imperceptible 

 when low powers are used, is sufficient to 

 render the most perfect optical arrangements 

 for high magnifying powers next to useless. 

 That the whole instrument should be secured 

 as much as possible from vibration, by being 

 placed upon a steady table, and this on a 

 steady floor, is of the first importance for its 

 advantageous employment. laut, as an entire 

 freedom from this cause of vibration can rarely 

 be obtained in an ordinarily-constructed house, 

 the next point to be aimed at is such an ar- 

 rangement of the optical portion of the instru- 

 ment in regard to the object, that any vibra- 

 tion which takes place may affect both alike, 

 in which case it will be scarcely perceptible. 

 In many microscopes of ordinary construction 

 the stage is comparatively motionless, whilst 

 the body (containing all the optical portion) 

 is subject to tremor; and in such instruments, 

 when a high magnifying power is employed, 

 the object is seen to oscillate so rapidly upon 

 the slightest cause of vibration (such as a per- 

 son walking across the room or a carriage rol- 

 ling by in the street) that it is frequently almost 

 indistinguishable. Various modes have been 

 devised for obviating this inconvenience, the 

 chief of which we shall hereafter notice. 



2. Capability of accurate adjustment to 

 every variety of focal distance, without move- 

 ment of the object. It is now a principle 

 almost universally recognised in the construc- 

 tion of good microscopes, that the stage on 

 which the object is placed should be a fixture; 

 and that the movement by which the focus is 

 to be adjusted should be effected in the body 

 or optical portion. Several reasons concur to 

 establish this principle, which was, we believe, 

 first insisted upon by Dr. Goring.* Among 

 the most important we consider to be that, if 

 the stage is made the moveable part, the ad- 

 justment of the illuminating apparatus must 

 be made afresh for every change of magnify- 

 ing power, whilst, if the stage is a fixture, the 

 illumination having been once well adapted, 

 the object may be examined under a great 

 variety of magnifying powers, without its being 

 changed in any respect. Moreover, if the 

 stage is the moveable part, it can never have 

 that firmness given to it, which it ought for 

 many purposes to possess. It is almost im- 

 possible to make a moveable stage free from 

 some degree of spring; so that, when the hands 

 bear upon it in adjusting the position of an 

 object, it yields to an amount which, however 

 trifling, becomes apparent with high powers 

 by the alteration of the focus. We might add 

 many more reasons, but these will here suffice. 

 It having been determined, then, that focal 

 adjustment should take place in the optical 

 portion of the microscope, the next point for 

 consideration is the mode of effecting it. This 

 should be such as to allow free range from a 

 minute fraction of an inch to three or four 



* See Pritchard and Goring's Microscopic Illus- 

 trations. 



inches, with equal power of obtaining a deli- 

 cate adjustment at any part. It should also 

 be so accurate that the 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 Us field, 

 notwithstanding the great alteration in the 

 focus. In this way much time may often be 

 saved by employing a low power as ajinder 

 for an object to be examined by a higher one; 

 and, when an object is being viewed by a suc- 

 cession of powers, no readjustment of its place 

 on the stage is required, such as would other- 

 wise be necessary for each. The best modes 

 of securing these ends, also, will be considered 

 in their proper place. 



3. The power of placing the instrument 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 posi- 

 tion as to be inconvenient to the observer. It 

 is certainly a matter of surprise that opticians 

 should have so long neglected the very simple 

 means which are at present so commonly em- 

 ployed, of giving an inclined position to mi- 

 croscopes ; since it is now universally acknow- 

 ledged that the vertical position is, of all that 

 can be adopted, the very wont. We do not 

 ourselves consider the horizontal position of 

 the body and its appendages at all an advan- 

 tageous one, although it has been adopted in 

 some of M. Chevalier's latest and best instru- 

 ments. In the first place it requires that the 

 whole microscope should be raised so much 

 above the level of an ordinary table, as to 

 bring the eye-piece to the height of the eye of 

 the observer when sitting upright at his ease ; 

 if this be not done, a constrained and conse- 

 quently disadvantageous position of the head 

 is required on his part; and if it be, all the 

 manipulations must be executed at an elevation 

 very inconvenient and fatiguing to the arms. 

 Moreover either the stage must be rendered 

 vertical, in which case all the objects must be 

 so secured (to prevent their slipping) as to 

 render the necessary movement of them very 

 difficult; or, the stage being horizontal, the 

 direction of the rays must be changed, after 

 they have passed through the object-glass, by 

 a prism or a mirror placed at an angle of 45 

 in their course, as in M. Chevalier's construc- 

 tion, which we think a decided disadvantage, 

 as introducing another source of imperfection 

 and error. We believe that it will be gene- 

 rally acknowledged, that an inclination of about 

 45 to the horizon is the most convenient for 

 unconstrained observation ; and the instrument 

 should be so arranged, that, at such an incli- 

 nation, the stage may be so far elevated above 

 the table, that, when the hands are employed 

 at it, the elbows may rest 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 



