292 HOW TO WORK 



follow. I do not for one moment pretend that it is equally appli- 

 cable to all tissues, or that it will succeed in all hands ; but I am confi- 

 dent that it is based upon principles of the utmost importance for 

 successful demonstration. Every year I myself discover improve- 

 ments in detail of the utmost advantage ; but the basis of the process 

 remains the same, and, as I have now been actively engaged in 

 minute microscopical investigation for twenty years, it is scarcely pos- 

 sible that principles which have been adhered to so long, can be desti- 

 tute of advantages. Moreover, in the hands of some of my pupils 

 it has answered as well as in my own.* 



364. Conditions to be fulfilled in Demonstrating Minute Struc- 

 ture by the Highest Powers : 



1. Of many tissues, sections sufficiently thin for high powers 

 cannot be obtained by the processes usually adopted. In order to 

 make the specimen thin enough, pressure must be employed, and in 

 many instances very strong pressure is required. Even by very 

 moderate pressure, tissues immersed in water are destroyed com- 

 pletely, and experience has proved that the requisite amount of 

 pressure can only be employed if the tissue be immersed in, and 

 thoroughly impregnated with, a risen! medium, which is not only 

 readily miscible with water in all proportions, but with such chemical 

 reagents as may be required to act upon one or more constituents of 

 the tissue for the purposes of demonstration. 



2. As many structures are exceedingly .delicate, and undergo 

 change very soon after death, it is necessary that the medium in 

 which they are examined should have the property of preventing 

 softening and disintegration, and should act the part of a preservative 

 fluid. 



3. In order that tissues should be uniformly permeated with a 

 fluid within a very short time after the death of the animal, it is 

 necessary that the fluid should come quickly in contact with every 

 part of the texture. This may be effected in two ways : 



a. By soaking veiy thin pieces in the fluid, or 



b. By injecting the fluid into the vessels of the animal. 



4. As different structures require fluids of different refractive 

 power for their demonstration, the medium employed must be such 

 that its refractive power can be increased or diminished, unless for 



* An excellent illustration of the great importance of careful preparation is 

 afforded by the reply of Mr. Gedge, of Cambridge, to the observations of 

 I )r. Moxon, concerning the distribution of nerve fibres to the muscles of a culex 

 larva. Microscopical Journal, July 1867, p. 193. 



