i6 



SCIENTIFIC NEVV^S. 



[March ist, 1887. 



ORTHOCHROMATIC PHOTOGRAPHY. 



THE ready-prepared sensitive " dry plate " of commerce 

 is a notable production of chemistry. Previous to its 

 introduction, photographers prepared their sensitive films 

 or plates immediately before using them, a process at 

 once costly, unclean, and uncertain. It involved the use of 

 cumbersome apparatus, and the plates when prepared 

 lacked the desired sensitiveness; but all this has been 

 changed by the use of dry plates, as they can be made of 

 any desired sensitiveness, and, generally speaking, are of ex- 

 cellent quality. There is, however, still this one great defect 

 in photography ; it represents certain colours in a manner 

 quite opposite to the effect they produce on the visual 

 organs. Being now familiar with the ordinary photographic 

 representations of colour, we do not realise this reversal of 

 tones with the same force, perhaps, as those who in the 

 early days of photography nick-named it the " black art." 

 It is, however, none the less true, that pale 3'ellows, for 



Fig. I. 



instance, are represented as though they were dark colours. 

 Blues, on the other hand, appear in comparatively light 

 shades. Dark blue is represented by the ordinary photo- 

 graphic plate as though it were a lighter colour than a 

 lemon ; and this under precisely similar conditions of 

 exposure. There is, in fact, a great need of a sensitive 

 plate capable of representing all colours in their true scale 

 of tones. Many attempts have been made to produce such 

 plates. They are generally known as " Orthochromatic," 

 or direct-colour plates. 



Mr. W. H. Hyslop recently read a paper on this sub- 

 ject at a Photographic Conference held at the Society 

 of Arts, and he then exhibited some very interesting 

 diagrams, showing the different colour-effects produced 

 by dry plates differently prepared, but exposed to light 

 under precisely similar conditions. Having permission to 

 reproduce these diagrams we have selected the two most 

 typical, and by their aid we trust to make clear the advan- 

 tage of the orthochromatic system. Fig. i is a photograph 

 of bands of different colours taken with an ordinary 



bromide of silver plate, the natural colours of the various 

 stripes being as follows : — D R, dark red ; L R, light red ; 

 Bl., black ; L, lemon ; Or., orange ; D B, dark blue ; L B, 

 light blue; W, white ; Gr., green; Vio., violet. 



It will be seen at once that the effects produced on the 

 photograph are nearly the reverse of those seen in the 

 object photographed. Fig. 2, on the other hand, shows a 

 much nearer approximation to right shades, and has been 

 produced by using an orthochromatic plate, in the preparation 

 of which erythrosine has been introduced, and by placing 

 a faint yellow glass screen between the object and the 

 plate. 



It will be noticed that the lemon colour, which appears 

 dark in Fig. i, is much brighter and more natural in Fig. 2. 

 In the orange band the effect is still more striking. Violet 

 we naturally regard as a moderately dark colour ; but 

 photography has hitherto represented it as one of the 

 lightest. To understand the reason of this let us, for a 

 moment, consider the relation to photography of the three 



Fig. 3. 



great divisions of the spectrum, viz., the chemical, visual, 

 and heat rays. Photography is essentially a chemical 

 process, the chemical and ultra-chemical rays affecting 

 vigorously the sensitive plate. The visual rays have con- 

 siderably less effect ; the heat rays have, comparatively, 

 none. If then we place the object shown in our diagram 

 side by side with the spectrum, we can at once see the cause 

 of the reversal of tones in the photograph. The violet 

 band reflects the chemical rays, and the result is a strong 

 impression upon the plate. Hence a light colour is shown 

 on the finished print. The opposite of this is seen in the 

 case of the reds, and undoubtedly red is to us a very striking 

 colour. Photography, however, requires rays having the 

 necessary chemical properties, and of these red has little or 

 none. Practically no eff'ect is produced when the sensitive 

 plate is exposed to the red rays, and the finished print 

 therefore shows red as a dark colour. Yellow may be 

 shown to have an intermediate effect, but the examples 

 given are sufficient to explain the general principle. 



The very important question then presents itself: In 



