This is just as true of all the shades and hues as it is of the three fundamental colours, for the various rays of light will penetrate the starch in just the proportion of the hues they represent in the scene before our eyes. While the silver solvent will remove the dark images built up by the penetration of green light, it will leave behind the particles of red-orange, and blue-violet, backed up by the creamy silver bromide of the emulsion. If above the green field we had a blue sky, the blue-violet particles would let the blue-violet rays penetrate them, and record the image of the sky. 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 Water 4 ounces Metoquinone stock solution 2 drams