HISTOLOGICAL SUBJECTS. 213 



Pure ether and strong alcohol, equal parts ; for each ounce of 

 this mixture weigh out five grains of ammonium iodide, two 

 grains of potassium bromide, and five grains of photographic 

 pyroxylin. Dissolve the salts in the smallest possible amount of 

 water, and add to the alcohol. Dissolve the pyroxylin in the ether. 

 Put the two solutions together, and filter. The collodion is now 

 ready for immediate use. It should be made in small quantities, 

 since it cannot be relied on to keep longer than three weeks. The 

 sensitising bath is, as usual, forty grains of silver nitrate to the 

 ounce of water. Ferrous sulphate developer works well, and is 

 easily prepared : — Saturated solution of ferrous sulphate (not aci- 

 dulated with sulphuric acid), four ounces ; glacial acetic acid, one 

 ounce and a half, to sixteen ounces of water. After fixing in 

 potassium cyanide, the negatives may be set aside for a more con- 

 venient time, or intensified at once. The plate must be either 

 entirely wet or perfectly dry before commencing, otherwise the 

 action will be uneven. There are several methods of intensifying 

 negatives which are useful for photo-micrographs. 



I. — Pay the negative in a tray containing a watery solution of 

 .iodine and iodide of potassium, or, what is more convenient, pour 

 on and off a strong watery solution of a deep wine colour, until 

 the negative assumes throughout a delicate straw colour. Then 

 wash very thoroughly to eliminate the iodine. This process may 

 be hastened by pouring on a very dilute solution of potassium 

 iodide. The fixing agent (cyanide) should have been carefully 

 washed out, otherwise as fast as the deposited silver is changed 

 into an iodide it will be dissolved as a cyanide of silver, and the 

 image is lost. If the iodine is not washed out, it reacts with the 

 sulphur next used, and produces a disagreeable yellowish-green 

 colour, which interferes with the printing qualities of the negative. 

 Finally, treat the plate with a soluble sulphide; Schlippe's salt has 

 been used, but ammonium sulphide is the most satisfactor)-. Pour 

 this on and off until the film is grey to the back ; this insures that 

 all the deposited silver is changed to a sulphide. When the nega- 

 tive is dried and varnished, it is ready to be printed from. This 

 process gives a good deal of density, and is adapted to negatives 

 from objectives of one-fifth of an inch and upward, where the 

 exposure, being so short in order to give good definition, neces- 



