Early Work In Photography
A Text-book For Beginners

CHAPTER IV. THE DARK-ROOM AND ITS FITTINGS. 40 ARK-ROOM is a misnomer, but we will try to explain |I why such a chamber, in which the critical operation . of development is usually carried on, is designated “dark.” . , . Most students are aware that white light, when examined through the spectroscope, is seen to consist of many beautiful and vivid colors. Now one end of the spectrum is princi pally formed of blue and violet light, while the constituents of the other end are chiefly yellow and red. , :, It is well known (even among the general public) that red cannot be photographed so quickly as can blue—hence it is also well known that a bright red dress usually appears black in a photogram, while blue, even dark blue, comes out white. Now we will try to show the reason for this. White light contains both rays of light and rays of heat. 1he spec- troscope divides this light into its several parts and shows us that it is possible to so filter white light as to exclude the active rays of light at will. The blue and violet rays are the actinic* ones, and exert the greatest power upon a photographic plate, of which we shall speak later. The red or heat rays are the non-actinick ones and exert no influence on a light-sensitive plate. Photographic plates are now prepared to such a degree of sensitiveness that an exposure to white light of even a small fraction of a second is sufficient to impress it with a visible image, but the same plate may be exposed to red and yellow light for considerable time without leaving any sign of an impression. At the risk of repetition we once more impress upon tlie student the reason for this: the red and yellow rays transmit only heat; the blue and violet rays transmit active light. How, then, can we so illuminate a chamber that we can conduct photographic operations without risk of injuring so sensitive a plate? Easily enough. We have said that it is possible to filter certain rays from others, and it is only * See Glossary.