No. 6, Aurantia (single) —This was photographed through a screen made by coating thin mirror plate glass twice with the following colored collodion : Aurantia 0.3 grammes Warm alcohol 25. c.c. Collodion (2 per cent.) 5o. c.c.
This prints shows the yellows and greens lighter and the blues darker than in No. 1.
No. 13, Aurantia (double)—This was taken through two screens like No. 6 placed face to face. It gives a nearly perfect reproduction of the relative brightness of the different colors with the exception, perhaps, of the red being a little too dark. Comparison of this with No. 6 shows that even with a dye possessing a power of absorbing the so-called actinic rays, a sufficient body must be used to obtain good results.
No. 8, Naphthol Yellow (single).—A 31 x 3 piece of mirror plate was leveled and coated as thick as possible with a warm solution of Naphthol yellow o.5 grammes Hard gelatine to. grammes Water 200. C.C.
Reasoning from analogy, one would say that if, as in No. 13, two screens were used, a considera bly better result would be obtained, but experience has proved otherwise. Two screens while increasing the exposure considerably, gave but a slight improvement.
No. 14, Chrysoidine (double).—This reproduction is shown in order to illustrate the unre liability of the ordinary pocket spectroscope as a test for a suitable screen dye. Judging from an inspection with a pocket spectroscope this screen should give as good, if not better, results than No. 13, but a comparison of the two plates will show it that gives a more false rendering than even No. r (without a screen).
The following is the formula used : Chrysoidine .5 grammes Alcohol 50. c.c. Ether.... 5o. c.c. Pyroxyline 2. grammes
Thin mirror plate glass was coated once with this and two screens were placed face to face' They allowed so little light to pass that it was difficult to focus. This shows that the suitability of a dye is of more importance than the quantity. If a single chrysoidine screen is used with the naphthol yellow screen with which No. 8 was made, very good results are obtained with a long exposure.
The lengthening of the exposure by the addition of the naphthol yellow screen to the chry soidine screen seems to be out of proportion to the amount the image is darkened on the ground glass, which fact is explained by Mr. Ives' showing that the ultra violet rays play a more impor tant part in the production of an ordinary negative than is generally supposed, and it is these rays which the naphthol yellow screen absorbs, but which are allowed to pass by the chrysoidine screen, although the latter absorbs more of the visual rays.
Too short an exposure was mentioned as the rock upon which explorers were apt to strike and proclaim that there was no free channel. That one could err on this point is easy to be seen, for while with an orthochromatic plate the use of the double aurantia screen would lengthen the exposure from 20 to 3o times, I should judge, I found that with an ordinary plate and a bright sunlit landscape, this screen increased the exposure at least one thousand times. This was accomplished by exposing for minutes instead of seconds and using the largest stop allowable.
Of course such an exposure would restrict the use of an ordinary plate with a color screen to the photographing of distant views and to inanimate or practically inanimate objects.