M. Tranchant in his article first deals with the subject of orthochromatism by means of col oring baths. Referring to the fact that plates prepared by the processes recommended by Vogel, Mallmann, and Scolik, do not keep for any length of time, he gives a method which he declares has not this inconvenience attached to it. Supposing that in dyeing to each argentic molecule a colored molecule is attached, which absorbs certain radiations, the natural necessity is that this substance shall not alter the silver bromide or gelatine. It is for this reason that only the follow ing colors can be employed.
Eosine, erythrosine, cyanine, croculeine, and azuline.
Apparently croculeine would be the most complete substance, but it is not much employed, owing to its so often being of inferior quality and difficult to obtain commercially.
All plates thus prepared do not keep well, owing, says the author. to the alcohol and am monia added to the bath. We think it to be more likely to be due to the latter substance, however, unless to impurities in the alcohol.
Buy the eosine soluble in water ; as to the azuline a little rectified alcohol at 90 deg. should be added, or, what is better, dissolve the azuline in alcohol, add the water, boil the solu tion in a porcelain vessel and allow it to cool.
For use, pour the color bath, after having filtered it, into a porcelain tray, and allow the plate to absorb the liquid for one minute ; drain and set it aside to dry in the dark at the tem perature of 25 deg. C. (73.4 Fahr.).
All these operations should be made by the red light of the dark-room, and the plates carefully guarded, even from this light, when not actually required. Plates prepared in this man ner are said to be very good and keep well for a long time. They have but one defect : the ex posure should be three times longer with the azuline plates. It is for this reason advice is given to prepare them only with extra rapid plates. Moreover, the latter plates, whether they be orthochromatized by azuline (which renders them very sensitive to green and yellow, a little less to red), or prepared with eosine (which does not render the red), must, in cer tain cases, be completed by the use of a screen— one of a light green color.
Referring to orthochromatism by colored screens the author says that all the plates of com merce, and even those prepared with eosine, erythrosine and azuline, should, in order to render the reds, be exposed for at least half the time of exposure by a light green screen.
Those prepared for yellow and green should be completed for the green by a screen tinted with rose.eosine.
It is well known that each color has a complementary one ; project a beam of solar light through two glasses selected with care ; for example, a green and red glass, and if the light filtered, as it were, through these glasses is white, it is certain that the green superposed to the red is the complementary color of this red. It is, however, absolutely impossible to obtain colors so pure that the mixture will produce white. A dirty gray is the nearest Approach to this effect that can be obtained.
But as there are only six fundamental colors from which all the others are formed, there will only be six useful screens : Red Green Yellow Blue Violet Orange The orange is composed of yellow and red, and the violet of blue and red, hence these two last screens are not required. There remain therefore four screens : red, green, blue and yellow, which must be arranged two by two. But in most cases three screens are sufficient, thus : Red for green 2 Green for red Blue for yellow To obtain all the tints of a landscape, these three screens should succeed one another dur ing an exposure, proportionate to the numbers indicated, viz.: 2 X seconds for the red, i x x seconds for the green, and z x x seconds for the yellow. It suffices to prepare them beforehand in a manner sufficiently transparent, but nevertheless colored enough to obtain the result desired.
To prepare the screens faultless pieces of glass should be selected, colored light green, light red, and light blue. They should be rounded in a shape ena bling them to be placed in the tube of the lens.
It is very difficult to find colored glass of the required quality, which must be of a pure color, and having per fectly plane sides; therefore, it is, as a rule, substituted: z. By an ordinary lass covered with a transparent film containing the colored me dium.
2. By a pellicle or film of collodion, or of celluloid containing the color.
For the first method very fine and thin glasses are cut into shape, and coated with a small quantity of collodion very lightly colored with eosine. It is of importance that the film be very equal. We advise using a very thin collodion, and coating twice, draining the superfluous collodion from opposite corners, otherwise there is always a tendency for the collodion to be thin at the top and thick at the parts where the solution was poured off.