After the negative has been treated and made a positive, a second development reduces the silver bromide to opaque metallic silver, preventing any light from passing through the grains through which a part of the image did not pass. This second bath also brightens the colours, while the hypo bath removes the unaltered silver bromide ensuring permanency to the image.
"Of course in taking these colour photographs," went on the scientist, "we must take into consideration a great many things, to which the manufacturers will call your attention in their booklets. The exposure is the most important part of all, for these plates are necessarily slow and must be exposed for a much longer time than the ordinary rapid plates. For instance, this field, with this bright summer sunlight, will require a full second with this lens at U. S. 4."
The scientist then went on to give the boy directions for developing his colour plates, as follows:
The whole process of development consists of three operations and but two solutions are required, one of them being kept preferably in two stock solutions. Apothecary weight is used.
STOCK DEVELOPER
| Water | 30 ounces |
| Metoquinone | 3-1/2 drams |
| Sodium sulphite (dry) | 3 ounces |
| Ammonia (density 0.923 or 22 degrees B) | 1 ounce |
| Potassium bromide | 1-1/2 drams |
Dissolve the metoquinone first in lukewarm water and then the other chemicals in the order given.
STOCK REVERSING SOLUTIONS
| A. Water | 25 ounces |
| Potassium permanganate | 50 grains |
| B. Water | 25 ounces |
| Sulphuric acid | 4 drams |
Errors in exposure are to be corrected by varying the duration of development and the amount of stock solution added after the appearance of the image. Use the solutions at a temperature of 60 degrees Fahrenheit, and start development of a 5 × 7 plate in