I02 Journal of Comparative Neurology. 



with bichromate of potassium or ammonium and silver nitrate. 

 (2) The method of the successive action of a mixture of osmic 

 acid and bichromate and of silver nitrate. (3) The method of 

 the combined action of bichromate of potassium or ammonium 

 and bichloride of mercury. (The stain appears black by trans- 

 mitted, metallic white by reflected light). 



"(i) The method of the combined actio) i of bichromate of 

 potassiian a)id of silver nitrate. In the series of methods which 

 I have especially employed this is, in a manner, the fundamen- 

 tal one. The others are only variations of this, devised to 

 shorten the time of the preliminary treatment, to make the 

 preparations more stable, to vary the results in various ways, 

 especially to obtain a greater extension of the reaction and to 

 cause the reaction to affect one or another species of the ele- 

 ments or a part of them. 



" I consider it to the point to call attention to the fact that 

 the procedure of the microscopical technique which I will de- 

 scribe, although it rests essentially upon the action of silver ni- 

 trate, has nothing in common with the usual method of staining 

 the intercellular substance of endothelium, epithelium and con- 

 nective tissue brown or black. In the latter method dilute so- 

 lutions of silver nitrate are applied immediately to the fresh 

 tissue, exclusively to the surface of membranes or membranous 

 tissues of slight thickness (aponeurotic plates, substance of the 

 cornea, intima of vessels) and light exerts an important influ- 

 ence upon the reaction whereby the blackening of the combina- 

 tion which the silver salt forms with the ground substance is 

 brought about. With my method the light has nothing to do 

 and the reaction takes place through the gradual penetration of 

 the silver salt into more or less voluminous pieces which have 

 been previously treated with bichromate. The black-staining 

 of the various elements composing the nervous tissue results 

 from a reducing action which the elements themselves exert, 

 under the influence of the bichromate, upon the silver salt. 



" The procedure necessary to bring about the black-stain- 

 ing of the elements of the central nervous system consists essen- 

 tially of two parts. 



