ILLUMINATION OF OPAQUE OBJECTS. 163 



object to produce the same effect. By modifying the distance of 

 the Condenser from the lamp and from the object respectively, the 

 cone of rays may be brought nearly to a focus, or it may be spread 

 almost equably over a large surface, as may be desired. And the same 

 effect may be produced by shifting the position of the Condenser, 

 when Beck's Parabolic Speculum is employed in combination with 

 it. No more effective illumination can be desired for objects viewed 

 under the low powers to which the Parabolic Speculum is adapted, 

 than that which is afforded by this combination ; the Bockett Lamp 

 (Fig. 85) supplying a most convenient means of using it, as the 

 Author can testify from a very large experience. In the illumina- 

 tion of Opaque objects, Artificial light has the advantage over 

 ordinary daylight of being more easily concentrated to the precise 

 degree, and of being more readily made to fall in the precise direc- 

 tion, that may be found most advantageous. Moreover, the con- 

 trast of light and shadow will be more strongly marked when no 

 light fails upon the object except that proceeding from the lamp 

 used for its illumination, than it can be when the shadows are par- 

 tially lightened by the rays which fall upon the object from every 

 quarter, as must be the case if it be viewed by daylight. If a 

 more concentrated light be required, the small Condensing Lens 

 may be used in combination with the Bull's-eye, being so placed as 

 to receive the cone projected by it, and to bring its rays to a more 

 exact convergence. In this manner very minute bodies may be 

 viewed as Opaque objects under high magnifying powers, provided 

 that the brasswork of the extremities of the Objectives be so 

 bevelled-off as to allow the illuminating cone to have access to the 

 object. As none but a very oblique illumination, however, can be 

 thus obtained, the view of the object will be by no means complete 

 unless it be supplemented by that which may be obtained by means 

 of the Vertical Illuminator (§ 93), which supplies for high powers 

 the kind of illumination that is given by the Lieberkiihn for the 

 lower. 



124. There are many Opaque objects which it is desirable to 

 view from all sides, in order that their features may be completely 

 made out. For such as can be conveniently attached to small 

 disks, Beck's Disk -holder (§ 95) affords by far the most convenient 

 and effective mode of presenting them in every variety of aspect. 

 Many small objects, such as the Capsules of Mosses, may be 

 grasped in the Stage-Forceps ; and by a little care in manipulation 

 every part may be brought into view successively. In either of 

 these cases the Lieberkiihn can be employed with powers that are 

 too high for the Parabolic Speculum ; and light of considerable 

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

 Mirror out of the axis, or by covering the greater part of the e- 

 flecting surface of the Lieberkiihn by means of a cap, or by a com- 

 bination of both methods. Whenever the Lieberkiihn is employed, 

 care must be taken that the direct light from the Mirror be entirely 



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