11 INTRODUCTION. 



the tissues, and the experimental investigation of their proper- 

 ties conducts us, in many instances, to a knowledge of the 

 most delicate structural arrangements; whilst the converse 

 always holds true, that a thorough knowledge of structure 

 furnishes the key to many vital phenomena. 



The technical methods of research applicable to these two 

 subjects are nevertheless different. When we desire to follow, 

 and ultimately to modify, the vital processes under the 

 microscope, other means of research are required than when 

 we merely wish to acquaint ourselves with the forms of the 

 elementary parts. Moreover, experiments which are performed 

 under the microscope differ according to whether they are con- 

 ducted on living or on dead bodies. The sensitiveness of the 

 former to external influences, makes even in the microscopi- 

 cally small compass of the instrument, and bearing in mind 

 the management necessary for its use experiments possible 

 under circumstances which are not practicable in the case of 

 dead tissue. Thus we find that changes can be induced in 

 living tissue by slight variations in temperature, by feeble 

 currents of electricity, and by weak solutions of acids ; whilst 

 the operation of these agents must be much more energetic 

 for the purpose of experiment on the dead body, and this is 

 not always agreeable for the observer, nor suitable for the 

 more delicately constructed instruments. The greater sensi- 

 tiveness of the living organism demands proportionate delicacy 

 in its treatment, but at the same time facilitates experiment ; 

 and to this we may ascribe the circumstance that experimen- 

 tation on the living body has gained so much in value during 

 the last few years, that is, during the period that the investi- 

 gation of living tissues has been so extensively undertaken. 



The tissues may either be examined by the light which they 

 reflect from their surface, or by that which passes through 

 them by direct or by transmitted light. Every object can be 

 examined by direct light, provided that the degree of illumina- 

 tion from without, and its own power of reflecting light, are 

 sufficiently great, and that both the object and the instrument 

 can be fixed. 



It is self-evident that the instrument must be capable of 

 \)eing focussed, or it would be impossible for trustworthy reti- 



