MANAGEMENT OF THE MICROSCOPE. 149 



here, too, the best position will be ascertained by trial. It will often be 

 desirable also to vary both the obliquity of the light and the direction 

 in which it falls upon the object; the aspect of which is greatly affected 

 by the manner in which the shadows are projected upon its surface, and 

 in which the lights are reflected from the various points of it. Many 

 objects, indeed, which are distinguished by their striking appearance 

 when the light falls upon them on one side, are entirely destitute both 

 of brilliancy of color and of sharpness of outline when illuminated from 

 the opposite side. Hence it is always desirable to try the effect of chang- 

 ing the position of the object; which, if it be 'mounted/ may be first 

 shifted by merely reversing the place of the two ends of the slide, and then, 

 if this be not satisfactory, may be more completely as well as more gradu- 

 ally altered by making the object-platform itself to rotate. With regard 

 to the obliquity of the illuminating rays, it is well to remark, that if the 

 object be ' mounted ' under a glass cover, and the incident rays fall at 

 too great an angle with the perpendicular, a large proportion of them 

 will be reflected, and the brilliancy of the object will be greatly impaired; 

 and hence when Opaque objects are being examined under high powers 

 with a very oblique illuminating pencil, they should always be uncovered. 

 149. The same general arrangement must be made when Artificial 

 light is used for the illumination of Opaque objects; the Lamp being 

 placed in such a position in regard to the Stage that its rays may fall in 

 the direction indicated in Fig. 114, and these rays being collected and 

 concentrated by the Condenser, as already directed. Since the rays pro- 

 ceeding from a lamp within a short distance are already diverging, they 

 will not be brought by the Condenser to such speedy convergence as are 

 the parallel rays of daylight; and it must, therefore, be farther removed 

 from the 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 the 

 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. 108) supplying a 

 most convenient means of using it, as the Author can testify from a very 

 large experience. In the illumination of Opaque objects, Artificial light 

 has the advantage over ordinary daylight, of being more easily concen- 

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

 the precise direction, that may be found most advantageous. Moreover, 

 the contrast of light and shadow will be more strongly marked when no 

 light falls upon the object except that proceeding from the Lamp used 

 for its illumination, than it can be when th'e shadows are partially light- 

 ened 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 flame of the lamp may be turned edgewise to the object, 

 and 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. It was in this way that Mr. 

 Beck obtained the views of the Podura-scnle given in plate II., Figs. 4, 

 5. In this manner very minute bodies may be viewed as Opaque objects 

 under high magnifying powers, provided that the brasswork of the ex- 

 tremities of the Objectives be so bevelled-off us to allow the illuminating 



