OTHER METHODS. . 117 



corneas (rabbit,, guinea-pig) require half to one hour, human 

 corneae two to five hours (in an acidulated solution). It 

 is better to err on the side of too-prolonged immersion, 

 rather than the contrary. In order to demonstrate the intra- 

 epithelial ramifications of nerves, the gold is partially reduced 

 by exposure for sixteen to twenty-four hours in (1 or 2 

 ounces of) distilled water, and there is added to the water 

 one or two drops of a pyrogallic-acid developing solution, 

 such as is used in photography (vide GEELACH, Die Photographie 

 als Hulfsmittel der mikroskopischen Forschung, Leipzig, 1863). 

 Or instead of treating them with the developing solution, the 

 corneae may be removed to a warm concentrated solution of 

 tartaric acid and remain there at the temperature of an incu- 

 bating stove until the gold is fully reduced. 



I have myself used the double chloride of gold and sodium, 

 with good results. 



CIACCIO (Journ. de Microgr., vii, 1883, p. 38; Journ. Roy. 

 Mic. Soc. (N. S), iii, 1883, p. 290) prefers the double chloride 

 of gold and cadmium. 



GEELACH, whose preparations of nerve-centres are said not 

 to have been equalled since, proceeded as follows (Strieker's 

 Handb., 1872, p. 678) : Spinal cord is hardened for fifteen to 

 twenty days in a 1 to 2 per cent, solution of bichromate of 

 ammonia. Thin sections are made and thrown into a solution 

 of 1 part of double chloride of gold and potassium to 10,000 

 parts water, which is very slightly acidulated with HC1. They 

 remain there from ten to twelve hours, and having become 

 slightly violet, are washed in hydrochloric acid of 1 to 2 : 3000 

 strength, then brought for ten minutes into a mixture of 1 

 part HC1 to 1000 parts of 60 per cent, alcohol, then for a few 

 minutes into absolute alcohol, and thence into clove oil, for 

 mounting in balsam. 



FLECHSIG (Die Leitungslahnen im Gehirn, 1876; Arch. f. 

 Anat. u. Phys., 1884, p. 453) reduces in a 10 per cent, solution 

 of caustic soda. 



NESTEEOFFSKY treats impregnated preparations with a drop 

 of sulphhydrate of ammonium, and finishes the reduction in 

 glycerin (quoted from Gierke's Farberei z. mik. Zwecken). 



BOHM reduces in Pritchard's solution. 



PEITCHAED'S SOLUTION consists of amyl alcohol, 1 per cent. ; 

 formic acid, 1 per cent. ; and water, 98 per cent. 



