The green glass is prepared in the same manner with green lumiere, and the blue glass with aniline blue.
With the second method on a glass plate is prepared for stripping, and the red collodion poured over it in the same manner as already recommended, then after stripping the film it is pasted on a cardboard diaphragm having the proper opening Eio, f/2o, 1/3o.
The green and blue screens can be prepared in the same manner.
To prepare triple and quadruple screens to reproduce all the tints the following method is given : A strip of glass is cut the width of the slide of a drop shutter and two-thirds longer. It is divided into five parts ; one is covered with black paper, and the second coated with a very thin film of white gelatine colored with eosine.
The third part should be colored with green lumiere by the same process ; the fourth with aniline blue, and the fifth on both sides covered with black paper. This screen must be held during the proper exposure in such a manner that the light passes successively through each colored part in order to obtain the colors of the original with their relative intensity.
The amateur can, however, modify the form of the screen as he thinks proper, provided it has the following qualities : r. Perfect opacity of the parts r and 5 covered with black paper.
2. Perfect transparency of the screens 2, 3, and 4 3. A separate stop for each screen is arranged in proper size in order to regulate the exposure.
In a recent issue of the PHOTO. TIMES Mr. Milton B Punnett discusses the question whether an ordinary plate can render correct color value, that is, reproduce in monotone the relative brightness of colors as they appear to the eye ? He says : " Many practical men such as F. E. Ives, Macfarlane Anderson and others claim it can, while among those who answer in the negative are men well versed in theory if not in practice. The writer, while mak ing experiments in defense of an assertion he had made, incidentally proved to his own satisfaction that some makes of ordinary plates possess orthochromatic properties to quite a pronounced degree, and has also been led to believe that all ordinary plates possess this quality to a greater or less extent.
The failure of others to obtain similar results can, without doubt, be attributed to one or more of the following reasons : First. To using a plate having very weak orthochromatic properties ; second, to using too light a screen or an unsuitable screen; third, to too short an exposure.
The Plate.—Why one make of plate shows stronger orthochromatic properties than another we will not discuss at length. It may be due to the method of making the emulsion or to the relative proportions of silver bromide and iodide in the film. That a film containing a large proportion of silver iodide is not as susceptible to the sensitizing influence of the dyet ordinarily used to impart orthochromatic properties is a well-known fact.
The Screen and Exposure.—For the sake of clearness in the following we will assume the spec trum to be divided, according to the old theory, into seven primary colors.
The sensitiveness of an orthochromatic plate to the yellow and green rays of the spectrum is so great that their value is often apparent without the aid of a medium to absorb the blue indigo, violet, and ultra violet.
With an ordinary plate whose sensitiveness to the red end of the spectrum is so weak compared with that of the violet end, a screen to absorb a portion of the rays of the violet end and thus to equalize the difference, is an absolute necessity, and, other things being equal, the darker the screen, the longer the exposure and the more pronounced the effect.
Mr. Ives has shown that a visual or photo spectroscopic test of a screen is not always reliable, and that a practical test is the only satisfactory method. A consideration of the reproductions shown herewith of photographs made on an ordinary Seed 26x 4 plate will more clearly explain the facts. The subject is a color plate in H. W. Vogel's Hand buch der Photooraphie.
No. 1, Without a Screen.—This shows the yel lows and greens too dark while the ultramarine blue is nearly as light as the pink.