VI INTRODUCTION. 



only those morphological elements are to be distinguished 

 which refract light differently from the surrounding parts, 

 and thus it happens that when fibres or cells are imbedded in 

 connective tissue, or in fluids, the refractive power of which is 

 the same as their own, they cannot be perceived even with the 

 best glasses, and artificial means must be resorted to in order 

 to render them visible. These may either be mechanical, 

 effecting the separation and isolation of the morphological 

 elements, or chemical, which dissolve the connecting material, 

 or act differently upon it than upon the morphological ele- 

 ments. The best artificially prepared specimens, however, 

 cannot supply the advantages of examination made on fresh 

 preparations with magnifying powers of from 1,000 to 1,500 

 linear. Those outlines which can be distinguished in the 

 living tissue, exhibit, besides sharpness, a certain softness, 

 which renders their definition easy and pleasant. The natu- 

 rally present cavities and fissures, in consequence of the different 

 refractive power of their contents, differentiate themselves 

 from the surrounding material with extraordinary sharp- 

 ness. Lastly, outlines are distinguishable during life, which 

 completely vanish after death. Even if these can be again 

 rendered visible by the application of peculiar reagents, their 

 full significance is only to be recognised by our knowing that 

 they are naturally present. 



In the present condition of our instrumental means of re- 

 search, it appears to be advantageous to commence histological 

 studies by means of general topographical examination of the 

 tissues with lenses of low powers ; then to proceed to the exami- 

 nation of specimens that have been subjected to manipulation 

 with lenses of moderate power, in such cases applying the 

 stronger lenses only as a means of control for the penetrating 

 powers of the weaker ones ; and finally to proceed to the exami- 

 nation of the fresh tissues with the best means at our command. 



I can attribute no very high value to the binocular (double 

 tubed) stereoscopic microscopes, so far as their use has at pre- 

 sent extended. As yet they have only been employed with 

 low powers. The relief of different parts of an object can be 

 very well ascertained, even with a simple microscope, by 

 merely varying the inclination of the head. 



