The advance of photography : its history and modern applications (1911)

46 THE ADVANCE OF PHOTOGRAPHY sensitive to blue. Bromide of silver is sensitive even to green, and iodide of silver only to violet and indigo. Mixtures of iodide and bromide of silver are sensitive both to blue and green. Dr H. Vogel succeeded, in the end of 1873, in making photographic plates sensitive even to those colours that were before considered to be inoperative — i.e. yellow, orange, and red. He found that if certain coloured substances that absorb green light were added to bromide of silver, which is by itself but slightly sensitive to green, the sensitiveness of this bromide to green is considerably increased. In like manner, the addition pf coloured substances absorbing yellow or red light makes bromide of silver sensitive to yellow and red light. After this discovery, it is not surprising that many of the difficulties attending the taking of coloured objects were soon overcome. We shall return to this point when considering the various photographic dry plates. Action of Ultra Violet Light upon Di-sulphate of Quinine. — Mention has often been made of the photography of the invisible. The case already recorded of the photographs of invisible pock-marks belongs to this. But the photography of invisible quinine writing is especially understood by the term, photography of the invisible. If a writing is made on paper with a concentrated solution of di-sulphate of quinine, the result is scarcely visible. If this is photographed, it appears black and plainly visible in the picture. The di-sulphate of quinine has the property of lowering the tone of violet, of ultra-violet and blue rays — that is, of converting them into rays of less refractive power and of less chemical effect ; therefore the light reflected from sulphate of quinine produces little or no photographic effect, and the written characters become black in the positive. This property of the di-sulphate of quinine serves also