THE TWO TYPES OF METHOD. 113 



abundantly evident that the very best gold preparations give 

 images that are only worthy of credence as to what they show, 

 and furnish absolutely no evidence whatever as to the non- 

 existence of anything 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 methode du chlorure d'or). 



The authors of some of the methods about to be described 

 claim for them that they give permanent preparations. I warn 

 the reader against indulging in the hope that, with all possible 

 precautions, his preparations will retain their beauty for more 

 than a few weeks. A successful gold preparation is certainly 

 a thing of beauty, but is exactly the opposite of a joy for ever. 

 The able histologist whose experience I have taken the liberty 

 of quoting above tells me that " as to permanence, they are 



"Like the snowfall on the river." 



205. The Two Types of Method. Gold methods may be 

 divided into two groups. The one, chiefly concerned with 

 the study of peripheral nerves or nerve-end-organs, is charac- 

 terised by employing either perfectly fresh tissues or tissues 

 that have been subjected to a special treatment by organic 

 acids. The other, concerned with the study of nerve-centres, 

 is characterised by the employment of tissues hardened in the 

 usual way. 



The hitherto classical rule, that for researches on nerve- 

 endings the tissues should be taken perfectly fresh, seems not 

 to be valid for all cases. For DEASCH (Sitzb. k. k. Acad. Wiss. 

 Wien, 1881, p. 171, and 1884, p. 516; and Abhand. math.- 

 phys. Cl. d. K. Sack. Ges. d. Wiss., xiv, No. 5, 1887; Zeit. f. 

 wiss. Mik., iv, 4, 1887, p. 492) finds that better results are 

 obtained with tissue that have been allowed to lie after 

 death for twelve, twenty-four, or even forty-eight hours in 

 a cool place. He even suspects that the function of the 

 organic acids in the methods inspired by Lowit's method, 

 is to bring the tissues into somewhat the state in which they 

 are naturally found at a certain moment of post-mortem 



8 



