CHAEACTEES OF GOLD IMPEEGNATIONS. 143 



you first impregnate very lightly with silver ; reduce ; treat 

 for a few minutes with a 0*5 per cent, solution of gold chloride, 

 and reduce in acidulated distilled water. 



This process, however, is in but little use, and except for 

 the staining of cytoplasm for cytological researches and for 

 certain special studies on the cornea, and on connective tissue, 

 the almost exclusive function of gold chloride is the impreg- 

 nation of nervous tissue. For this tissue, gold chloride 

 exhibits a remarkable selectivity, in virtue of which it justly 

 ranks as a most valuable reagent for the study of nerve end- 

 organs and the distribution of nerves. 



For all the objects above named gold chloride is capable of 

 furnishing preparations that for beauty and clearness cannot 

 be surpassed, if even they can be equalled by any other means. 

 A successful gold preparation shows at a glance, with dia- 

 grammatic clearness, a wealth of minute detail which perhaps 

 can only be painfully glimpsed by other means. But not 

 every gold preparation is successful. I think there is no use 

 in blinking the fact that very few are successful (one of the 

 most experienced authorities in the matter told me lately that, 

 as to nerve end-organs at all events, one preparation in ten 

 thousand is successful). I took up in the first edition of this 

 work the doubtless unpopular position that " with all possible 

 precautions gold chloride is uncertain in its action, and that 

 the results obtained by means of it need to be controlled by 

 the employment of other methods," and illustrated that posi- 

 tion at considerable length. 



That this position was the correct one is now generally 

 admitted. It is acknowledged to be abundantly evident that 

 the very best gold preparations give images that are only 

 worthy of credence as to what they show, and furnish abso- 

 lutely no evidence whatever as to the non-existence of any- 

 thing that they do not show ; for you can never be sure that 

 the imbibition of the salt has not capriciously failed, or its 

 reduction capriciously stopped at any point. That the images 

 frequently do stop capriciously short in the representation of 

 reality there is abundant evidence. One such case has been 

 treated by me ex professo in Recueil Zool. Suisse, i, 1884, p. 

 685 (Les organes chordotonaux des Dipteres, et la me'thode du 

 chlorure d'or). 



The authors of some of the methods about to be described 



