5<D THE MICROSCOPIC PREPARATION. 



small pieces and treated in the manner previously described. Instead of Muller's fluid, 

 that of Erlicki may be used, the time of treatment being then shorter (from five to eight 

 days). 



The objects may also be treated with a potassium bichromate-osmic acid solution 

 (2.5% solution of potassium bichromate, 8 vols. ; \% osmic acid, 2 vols.), the sections 

 thus treated being ready for immersion in silver nitrate after two or three days. It is ad- 

 visable to treat the objects with the potassium bichromate solution first, and then with the 

 potassium bichromate-osmic mixture. By this method the specimens remain under the 

 control of the investigator ; they may be examined either at once, or after an interval 

 varying between three or four and twenty-five to thirty days after immersion. If then one 

 or several pieces of tissue are taken, at intervals of two, three, or four days, from the potas- 

 sium bichromate solution and placed in the potassium bichromate-osmic acid mixture, 

 and then in the silver nitrate solution, various combinations of the fluids result, and the 

 investigator is usually rewarded with at least some sections giving most excellent 

 results. 



Another one of Golgi' s methods consists in successive treatment with potassium 

 bichromate and bichlorid of mercury. After remaining in the potassium bichromate for 

 from three to four weeks (a longer period is allowable), the objects are placed in a 0.25 % 

 to i% solution of corrosive sublimate. In the latter the specimens blacken much more 

 slowly than in the silver nitrate solution eight to ten days for smaller pieces ; for larger 

 ones, two months, and in some cases even longer. Before mounting the preparations in 

 glycerin or Canada balsam they must be carefully washed ; otherwise pin-shaped crystals 

 form within the sections and distort the whole view. The metallic white of the prepara- 

 tion may be changed to black by placing the celloidin section in a photographer's toning 

 solution consisting of : (a) sodium hyposulphite 175 g-* alum 2Ogm., ammonium sulpho- 

 cyanid 10 gm., sodium chlorid 40 gm., and water 1000 gm. (the mixture must stand for 

 eight days and then be filtered) ; (b) a I % gold chlorid solution. The specimen is 

 placed for a few minutes in a solution composed of 60 c.c. of a and 7 c.c. of b, washed 

 again in distilled water, dehydrated with alcohol, and mounted in Canada balsam. After 

 toning and washing, the sections may still be stained. 



Golgi' s methods are extremely inconstant in their results. When successful, how- 

 ever, only a few elements are blackened each time, an advantage not to be underesti- 

 mated ; for if all nerves should stain equally well, discrimination between the various 

 elements in the preparation would be very difficult. Neither are the same structures 

 always impregnated ; sometimes it is the ganglion cells and fibers, at other times the neu- 

 rogliar cells, and occasionally only the vessels. 



After the foregoing explanation of Golgi 's methods as applied by 

 himself, we shall append a description of these methods as modified and 

 employed at the present time (Ramon y Cajal, Kolliker, von Lenhossek 

 and others). 



Golgi's methods are classified as the slow, the mixed, and the rapid. 



The slow method requires a preliminary treatment. Pieces of tis- 

 sue from i to 2 cm. in diameter are placed for from three to five weeks 

 in a 2 <f c potassium bichromate solution ; they are then transferred for from 

 twenty-four to forty -eight hours to a 0.75% silver nitrate solution, or for 

 a much longer time to a o. 5 J C solution of corrosive sublimate. 



In the mixed method the specimens are allowed to remain for four 

 or five days in a 2 % aqueous potassium bichromate solution ; then for 

 from twenty-four to thirty hours in a mixture consisting of i% osmic 

 acid i vol., and 2% potassium bichromate solution 4 vols. They are 

 then treated with a 0.75% silver nitrate solution for one or two days 



When the rapid method is employed, the specimens are immedi- 

 ately placed in a mixture consisting of i vol. of i% osmic acid and 4 

 vols. of a 3.5% potassium bichromate solution, and, finally, for one or 

 two days in a 0.75% silver nitrate solution, to every 200 c.c. of which 

 one drop of formic acid has been added. 



When employing these methods, and more particularly the one last 

 described (which seems to be the most efficient), the following conditions 

 must be carefully observed : If possible, the material should be absolutely 



