382 THE AMERICAN MONTHLY [Dec. 



Microscopical Technique Applied to Histology. — XHI. 



From the French of M. Rene Boneval. 

 THE EYE. 

 {Co7icluded from Page 342). 



The Cornea. — Select the cornea of the frog, the rabbit, 

 or from man if you can get one perfectly fresh. 



Irri'pr eg nation with /Silver. — To make a negative impreg- 

 nation rub a stick of silver nitrate over a frog's cornea 

 left in place, then take out the eye and put it in distilled 

 water. 



With fine scissors remove the cornea, scrape both sur- 

 faces to remove the epithelium and mount in glycerine or 

 in balsam. A frog's cornea thus treated is left for two 

 or three days in distilled water; then mount it in glycerine 

 or in balsam. The intercellular spaces have become clear 

 and the cells are stained black. This is the positive im- 

 pregnation. 



Impregnation by Gold. — Put a frog's cornea in lemon 

 juice or in a weak solution of acetic acid (1 per cent) for 

 five or six minutes. After sponging ofi* with bibulous 

 paper transfer to a 1 per cent solution of chloride of gold 

 for seven or eight minutes. Transfer to the acetified 

 water. Usually the reduction is complete in twenty-four 

 hours and the cornea has a beautiful lilac tint. Scrape 

 off the epithelium and mount the cornea in glycerine. If 

 the thickness prevents observation, with a sharp razor 

 cut sections parallel with the surface and examine as the 

 entire cornea was examined. 



Fixed Cells. — Put a frog's cornea on a slide and expose 

 to the vapor of iodine. When colored brown, scrape off 

 the epithelium and examine with a high power. If not 

 sufficiently colored expose again to the iodine. The net- 

 work of fixed cells is colored with remarkable precision. 



Put the cornea of a frog, a rabbit, man, etc., in a satu- 

 rated aqueous solution of picric acid. Harden in gum 



