THIS is probably the most beautiful of all color processes from a theoretical standpoint and yet is also that which has been the least practised, mainly because one has to prepare the plates, exposures are very long, and there is no known means of reproducing the results. It has remained, therefore, purely a laboratory process.

The first suggestion as to the possibility of this process was made by W. Zenker in 1868; it was later suggested by Lord Rayleigh in 1887, and O. Wiener in 1890, but it was not until 1891 that G. Lippmann, of Paris, actually succeeded in obtaining a color photograph in this way.

In order that the subject may be fully grasped we must enter, even though but superficially, into an explanation of the rudimentary principles of light. A brief explanation has already been given of the dispersion of light and the occurrence of the Fraunhofer lines, and the following table gives the principal of these in the visible spectrum with their wave-lengths and the number of vibrations per second:

Fraunhofer Line

Wave-length

Vibrations per Second in Billions

A

7593.97

395

B

6867.38

437

Fraunhofer Line

Wave-length

Vibrations per Second in Billions

C

6562.96

457

D1

5896.08

509

D2

5890.13

509

E

5270.43

569

b1

5183.73

579

F

4861.43

617

G

4301.43

696

h

4101.84

731

H

3968.61

760

K

3933.86

763

The wave-lengths are given in Angstrom units (10-millionths of a millimeter.)

Beyond A lies the infra red, the invisible region of which we are sensible in the form of heat; while beyond K is the ultra-violet, by which the chief chemical actions are caused.

Light always proceeds in straight lines and is usually supposed to be a wave-like motion in a hypothetical medium which is called the ether. We may assume that the particles of the ether are so closely compacted that a disturbance or vibration of one must give rise to corresponding vibrations in neighboring particles. This being granted, there must be a certain time required for the transfer of the agitation from one particle to another. If we picture the ether particles as a series of beads closely strung together on a wire, we may crudely represent them by 1 in Fig. 21. If now a pull is given to the first particle at A the vibration will travel along the wire and the beads will vibrate to and fro on both sides of the plane AB and we may, assuming that the direction of the light is from left to right, represent what happens by 11 in Fig. 21, in which A'B' is the plane of equilibrium or rest, and a is the crest, b the trough of the wave; the distance of the particles a, b, c, from A'B' is called the amplitude of the wave.

The Lippmann Process Or Interference Heliochromy PracticalColorPhotography 23The Lippmann Process Or Interference Heliochromy PracticalColorPhotography 24The Lippmann Process Or Interference Heliochromy PracticalColorPhotography 25

Fig. 21.

A wave-length is the distance between any two points similarly situated as regards the equilibrium plane, so that A'D',C'B', or ac are each a complete wave-length, and obviously the midway points are half wave-lengths, thus A'C is the half wave-length of A'D'.

When a ray of light meets a reflecting surface it is thrown off according to a well-known law, that the angle of reflection is equal to the angle of incidence; and if a ray is incident normally to a surface, that is, at right angles to it, it is reflected back at the same angle and on the same path. We can thus represent the state of affairs by 111 in Fig. 21, and the incoming and outgoing waves would travel the same paths; but at the points abcd, which are obviously each half a wavelength apart, the pull on the ether particles would be in contrary directions, as shown by the arrows; therefore, as the forces are equal it is obvious that there is no movement and there can be no light, consequently no chemical action. The points abcd are called the "nodes" and the spaces in between the "loops," in which the ether particles may be considered to vibrate to and fro pendulum fashion. Such a system forms a series of "stationary" or "standing" waves, and it is clear that if such a system traverses a sensitive film, there would be, under proper conditions, chemical action only in the loops, and on development the metallic silver would be deposited at these places only. So, a series of laminae would be formed exactly half a wave-length apart, their distance of separation being dependent on the color of the incident light, as will be seen from the above table.

White light incident on such a laminary series of mirrors is reflected according to its wave-length; each zone reflecting only light of that color which has a wavelength corresponding to twice the distance of separation of the layers.

The fundamental basis of this process is that the sensitive film shall be transparent and in contact, during exposure, with a reflecting surface which returns the light on its incident path. For the reflecting surface, which must be in optical contact with the film, Lippmann chose mercury, which, as everyone knows, is liquid at ordinary temperatures and highly reflecting.

There are two methods by which we can prepare the sensitive film, either by the old Taupenot albumen process, which was first used by Lippmann, or by the gelatine emulsion method. This latter is faster but a little more trouble to make. For the albumen take:

Albumen 1000 ccm.

Potassium bromide, 10 % solution 43 ccm Ammonia 43 ccm.

Beat to a froth and allow to stand for twenty-four hours to liquefy, and then filter through glass wool or decant from the flocculent sediment. Glass plates should be thoroughly scoured with hot soda and water, well washed and dried, and polished with alcohol until they show a perfectly even film when breathed on. Ordinary glass is not as a rule thick enough, and it is more satisfactory to use white plate glass, about one sixteenth inch thick. As the film is extremely transparent and it is not easy to tell which side has been coated, it is as well to mark the wrong side with a diamond or the edge of a three-cornered file. It will also be found that as a rule one side of the glass is smoother than the other, and this smooth side should be coated; it is easy to detect the smoother side by holding the glass level with the eyes and glancing along it.