150 THE MICROSCOPE AND ITS REVELATIONS. 



cone to have access to the object. As none but a very oblique illumina- 

 tion, however, can be thus obtained, the view of the object will be by no 

 means complete, unless it be supplemented by that which maybe obtained 

 by means of the Vertical Illuminator ( 116), which supplies for high 

 powers the kind of illumination that is given by the Lieberkuhn for the 

 lower. 



150. There are many Opaque objects, such as Fbraminifera, wli h 

 it is desirable to view from all sides, in order that their features may be 

 completely made out. This may be readily done with objects mounted 

 in slides, when the Microscope is provided with the Zentmayer stage, by 

 inclining the stage to one side or the other (first taking care" that the ob- 

 ject is well secured upon it), and then giving rotation to the object-plat- 

 form. For such objects as can be conveniently attached to small disks, 

 Beck's Disk-holder (Fig. 94), affords by far the most convenient and effect- 

 ive mode of presenting them in every^ variety of aspect; but the disks may 

 also be held by attached pins, either in the Stage-forceps, or by the inser- 

 tion of the pins into the cork-box at its other end ( 118), a variety of 

 movements being given in either case by turning the forceps in its tube. 

 So, again, many small objects, such as parts of Insects, may be grasped 

 in the Stage-forceps itself, and by a little care in manipulation, each 

 aspect may be brought into view successively. In either of these cases, 

 the Lieberkuhn may be employed for their illumination; and light of con- 

 siderable obliquity may be obtained by its means, either by turning the 

 Mirror out of the axis, or by covering part of the reflecting surface of the 

 Lieberkuhn by a cap, or by a combination of both methods. Whenever 

 the Lieberkuhn is employed, care must be taken that the direct light, from 

 the Mirror is entirely stopped-out by the interposition of a ' dark well ' 

 or of a black disk, of such a size as to fill the field given by the partic- 

 ular Objective employed, but not to pass much beyond it. Opaque objects 

 that are permanently mounted either upon cardboard disks, or in the 

 slides specially provided for them, may be presented to the Microscope 

 in a considerable variety of directions by means of Morris's Object-holder 

 (Fig. 95); which, however, can only be employed with side-illumination. 

 If it be desired to make the most advantageous use of this appliance, ob- 

 jects mounted in slides should be so placed that the parts to be brought 

 into view by its tilting movement may look towards the long edges of the 

 slide ; since it is obvious that a much greater inclination may be given to 

 it in either of these directions, than in the direction of either of its ex- 

 tremities. 



151. Errors of Interpretation. The correctness of the conclusions 

 which the Microscopist will draw regarding the nature of any object, 

 from the visual appearances which it presents to him when examined in 

 the various modes now specified, will necessarily depend in a great degree 

 upon his previous experience in Microscopic observation, and upon his 

 knowledge of the class of bodies to which the particular specimen may 

 belong. Not only are observations of any kind liable to certain fallacies, 

 arising out of the previous notions which the observer may entertain in 

 regard to the constitution of the objects or the nature of the actions to 

 which his attention is directed, but even the most practised observer is 

 apt to take no note of such phenomena as his mind is not prepared to 

 appreciate. Errors and imperfections of this kind can only be corrected, 

 it is obvious, by general advance in scientific knowledge; but the history 

 of them affords a useful warning against hasty conclusions drawn from a 

 too cursory examination. If the history of almost any scientific investi- 



