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GENERAL METHODS OF INVESTIGATION, BY S. STRICKER. ill 



nal images of various objects to be obtained. The examination 

 of an object cannot be conducted by direct light with high 

 powers, because the employment of such powers necessitates 

 the close approximation of the lens to the object, whereby the 

 latter is covered, and its illumination prevented. It is, how- 

 ever, possible to apply here the principle of the ophthalmoscope, 

 and then this difficulty is overcome. 



Examinations conducted by means of direct light are greatly 

 assisted by direct illumination, or, what is still better, by throw- 

 ing a pencil of rays on the part of the object to be illuminated ; 

 details then frequently become apparent which can scarcely be 

 detected with the mere diffused light of day. 



If examinations by means of direct light are undertaken at 

 greater distances as when, for example, lower powers are em- 

 ployed, or when the objects are examined or are prepared in a 

 uid it is advantageous to use Briicke's doublet. This is 

 laced in the arm of a Nachets' or Hartnack's stand, and the 

 object is placed on the stage. The focussing can then be easily 

 accomplished with the unassisted hand by moving the lens. 

 This combination is very serviceable for preparations that have 

 been teased out with needles, as in the isolation of ganglion 

 cells and the separation of fine fibres. The object is in every 

 instance placed on an opaque ground : if it be dark, upon a 

 dull grey ; and if clear, upon an opaque black ground. The 

 object requiring to be isolated should in all instances be laid on 

 a slide of polished glass, beneath which again may be placed a 

 piece of dull white or black paper, as may be most convenient. 

 For the examination of larger portions of tissue in fluid, little 

 shells may be used, resting on a plane base, and having a 

 spherical hollow, resembling an ordinary glass salt-cellar. A 

 dull opaque ground may easily be obtained by covering the 

 surface of the glass with a thick layer of coloured wax or gutta- 

 percha, which has the advantage of enabling the objects to be 

 fixed in position by transfixion with needles. 



If it be required to bring the object into strong relief, in 

 order to examine the details of the surface, the lenses of Stein- 

 heil of Miinchen are especially to be recommended. It is 

 advantageous, however, to attach them to an arm moving on a 

 ball and socket joint, which again plays, horizontally and ver- 



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