428 CHAPTER XXXIV. 



hyde solution, and after twenty-four hours transfers to pure 

 3*5 per cent, bichromate for at least 2 days (retina), or 3 to 

 6 (central organs) . He finds that by this means precipitates 

 are almost entirely avoided. This I also find, but I seem to 

 get a too abundant impregnation of capillaries. 



G-EROTA (Intern. Monatyschr. Anat., xiii, 1896, p. 108) first 

 hardens (brain) for a week or two in 5 to 10 per cent, formol 

 solution, then puts small pieces for three to five days into 

 4 per cent, bichromate, then into the silver. 



Similarly BOLTON (Lancet, 1898, p. 218 ; Journ. Roy. Hie. 

 Sue., 1898,* p. 244). 



SCHREIBER (Anat. Anz., xiv, 1898, p. 275) obtained good 

 results (on appendages of Crustacea which were impervious 

 to the osmic mixture) with mixtures of five parts 2 '5 per 

 cent, bichromate to one of 4 per cent, formaldehyde, or one 

 part 2'5 per cent, bichromate to two of 5 per cent, formalde- 

 hyde, the specimens remaining for one day in the first, for 

 two days in the second. 



Similarly DUBOSCQ (Arch. z. Exper., 1899, p. 483), warm- 

 ing the mixture to 40 C. 



VAN GEHUCHTEN (in litt.), and other observers, have not 

 obtained good results with formaldehyde. 



KALLIUS (Encycl., p. 564) finds these mixtures good for 

 brain, but not so much so for other organs. 



818. Acetic Aldehyde. VASSALE and DONAGGIO (Monitore ZooL, 

 ItaL, vi, 1895, p. 82) harden pieces of at most 1 cm. in thickness for 

 fifteen to twenty days in a mixture of five parts of aldehyde with 100 of 

 3 to 4 per cent, bichromate, changing the fluid after a few days, as soon 

 as it has become dark. The rest as Golgi. 



819. Modifications of the Silver Impregation. RAMON Y 

 CAJAL (Rev. trim. Hist., No. 2, 1888, n6te) found the addition of a very 

 little formic acid to the silver bath facilitated reduction. According to 

 VAN GEHUCHTEN (La Cellule, vii, 1891, p. 83), 1 drop of the acid should 

 be added to 100 c.c. of the silver. But the practice is now generally 

 abandoned. 



BERKELEY (Johns Hopkins Hosp. Rep., vi, 1897, p. 1 ; Journ. Roy. 

 Mic. Soc., 1898, p. 242) impregnates, after hardening in the osmio- 

 bichromate, in a freshly prepared solution of two drops of 10 per cent, 

 phosphomolybic acid to 60 c.c. of 1 per cent, silver nitrate, which in 

 winter should be kept at a temperature of about 26 C. 



HILL (op. cit., 813) takes instead of silver nitrate a % per cent, solu- 

 tion of silver nitrite, with O'l per cent, of formic acid added. 



