ABSTRACT
The enormous growth of photography as a hobby has made hundreds of thousands, young and old, acquainted with the methods of taking, developing, toning, and fixing the impressions which rays of light make upon the sensitive plate. A liquid containing a solid in fine particles that a milky appearance is produced is called an emulsion, and the emulsion for photographic plates is prepared by mixing two solutions, containing: gelatine, ammonium bromide, and potassium iodide and silver nitrate and ammonia. A photograph obtained on a plate prepared in the way described, represents only approximately the lights and shades of the original, because the activity of the rays varies with the colour. The application of photography was not as simple as it appears at first sight, because the germs are extremely sensitive to light and heat. Incidentally it was observed that certain black lines appeared in the photograph which corresponded to no visible part of the apparatus.
