Photogen

“ Nycteris would return, putting her arm round him, or patting his cheek. "Take a few steps more. Every step away from the castle is clear gain. Lean harder on me. I am quite strong and well now."
So they went on. The piercing night-eyes of Nycteris descried not a few pairs of green ones gleaming like holes in the darkness, and many a round she made to keep far out of their way; but she never said to Photogen she saw them. ”

“ It should be clearly borne in mind that if we place luminous organisms, say bacteria or fungi, in an atmosphere devoid of oxygen and find that no light is produced, this may merely mean that certain functions of the cell are interfered with, including light production, but does not necessarily indicate that oxygen is actually used up in the photogenic process. ”

“ It will be evident from the nature of the process, that the lights and shadows of an object are reversed. That which is originally opaque will intercept the light, and consequently those parts of the photogenic paper will be least influenced by light, while any part of the object which is transparent, by admitting the light through it, will suffer the effect to be greater or less in exact proportion to its degree of transparency. The object wholly intercepting the light will show a white impression ”

“ On the other hand, we should expect photogenic reactions, in which some of the chemical energy is converted into radiant energy, to give off much more light the greater the reaction velocity. As reaction velocity increases so rapidly with temperature (Q10 = 2 to 3) , luminescence intensity should rapidly increase with increase in temperature. ”

“ Therefore, bioluminescence and chemiluminescence are similar phenomena and they differ from all the other forms of luminescence which we have considered. The light from luminous animals is due to the oxidation of some substance produced in their cells, and when we can write the structural formula of this photogenic substance and tell how the oxidation proceeds, the problem of light production in animals will be solved. ”

“ The eye is sensitive only to a very small range of period, while there exists in sunlight a range many times as great.
Those waves of shorter period than the violet end of the visible spectrum will be deviated even more than the violet, and will therefore be beyond the violet. They are called the ultra violet rays, and can easily be detected by means of their chemical activity. They cause a number of substances to glow, and therefore by coating the screen on which the spectrum is received with one of these substances, the violet end of the spectrum is extended by this glow. ”

“ When they impinge upon a metal refractory enough to bear their shock without fusion, they can raise it to a heat so white and luminous as to yield, when analyzed, all the colors of the spectrum. In this way the dark rays emitted by the incandescent carbons are converted into light rays of all colors. Still, so powerless are these invisible rays to excite vision that the eye has been placed at a focus competent to raise platinum-foil to bright redness without experiencing any visual impression. ”

“ Visible radiation × visual sensibility is spoken of as luminosity; (3) by the amount of light (expressed in candles) produced in relation to a given expenditure of energy or in relation to the cost of the energy expended. Thus, of the radiation emitted from an incandescent electric lamp only a small per cent. is light, the rest being heat and actinic rays. It is therefore very far from being 100 per cent. efficient. If there were no infra-red or ultra-violet in the radiation from an incandescent lamp its efficiency would be 100 per cent. if we disregarded visual sensibility. ”

“ We cannot, therefore, see objects whose diameter is less than about two-ten-thousandths of a millimeter. But the photographic plate is sensitive to shorter waves than the eye; these waves are known as the ultra-violet. By illuminating the microscope stage with ultra-violet light it therefore becomes possible to photograph objects so small that they must forever remain invisible to the naked eye, unless, indeed, the progress of human evolution brings with it increased sensitiveness to the shorter wave lengths. ”

“ The separating effected by the field-glass causes the blue rays to fall so much nearer the centre of the eye-glass, where, owing to its spherical figure, the refractive power is less than at the margin, so that spherical error of the eye-lens may be said to constitute a nearly equal balance to the chromatic dispersion of the field-lens, and the blue and red rays l′ and l′′ emerge nearly parallel, presenting a fairly good definition of a single point to the eye. ”

“ Radiant heat and ultra-violet rays are the ether waves nearest in wave-length and properties to light; X-rays and the waves responsible for wireless telegraphy appear to be similar waves further removed along the scale of wave-length.
Now in order to photograph an invisible object we require rays that (a) affect a photographic plate; (b) are capable of refraction by a lens; and (c) are invisible to the eye. The properties of the principal known rays concerned may be summarised as follows:
It appears, then, that ultra-violet rays are suitable for our purpose ”

“ If an observer looks into the eye-piece of a telescope he can discern only those heavenly bodies that send to the earth a certain minimum of light; but when a photographic plate is exposed for long periods there is a cumulative effect of light on the sensitive emulsion. That is, the long-continued impact of the light rays causes, little by little, a gradual change in the constitution of the sensitive silver salt. ”

“ The most active chemical change, you will perceive, is produced by the rays above the yellow a; viz. 4, 3, 2 and 1 (as at b) the green (4) being the least active, and the blue (3) and violet (1) rays the most so, the action still continuing far beyond the point b which is the end of the luminous image.
Suppose we wish to copy by the Daguerreotype, or Calotype process, any objects highly colored--blue, red and yellow, for instance predominating--the last of course reflects the most light, the blue the least ”

“ A photograph of the heavens will sometimes reveal something which the eye could not see. So, a photographic plate will sometimes catch the smaller particles as well as the largest stars, too far away to be seen.
“If you consider the light as moving in waves it is easier to understand what effect light waves have on these discoveries. Artificial light travels in waves farther apart than in the case of natural light. The waves of this kind of light are so far apart that the colloid or small microbe can lie between the waves and make no impression on the eye or on the photographic plate.” ”

“ Hence the colour of the light would appear pure violet, though it was originally deep red. This extreme case shows that the “last ray to disappear” when a light is gradually extinguished may be very different in colour from that of the original light, and when more usual light-mixtures are considered, such as those of sunlight and starlight, which appear nearly white to the eye, the “ last ray to disappear ” is found to be in the green, very near E in the spectrum. This result has two important bearings on the use of the wedge photometer. ”

“ But the waves from our incandescent carbon-points appeal to another sense than that of vision. They not only produce light, but heat, as a sensation. The magnified image of the carbon-points is now upon the screen; and with a suitable instrument the heating power of the rays which form that image might be readily demonstrated. In this case, however, the heat is spread over too large an area to be very intense. Drawing out the camera lens, and causing a movable screen to approach the lamp, the image is seen to become smaller and smaller ”

“ We only know it is a fact.
All these ether rays may produce heat. The high-frequency rays affect the photographic sensitive plate, and also produce another effect that is of especial interest in connection with the study of the Röntgen rays. They have the power of exciting a peculiar luminosity, or light, in certain substances, which are for that reason called fluorescent.
Electrical discharges in vacuum tubes have long been known as sources of radiations which produce heat and affect the eye. ”

“ If the radiant energy is light (visible or ultra-violet) we speak of photoluminescence, if cathode rays we have cathodoluminescence, if anode rays, anodoluminescence, and if X-rays (Röntgen rays) we have radioluminescence. Inasmuch as the α, β, and γ rays of radium correspond to the anode, cathode, and X-rays, respectively, radium radiation also produces luminescence in many kinds of material. If the material gives off the light only during the time it is radiated we speak of fluorescence; if the light persists we speak of phosphorescence. ”

“ Attempts have been made to concentrate solar light by means of a parabolic reflector or lens, and then to throw it through the microscope slide. The great drawback to this system is that intense heat is thrown upon the slide containing the organisms, and for these small creatures heat spells death. The same difficulties arise with acetylene and electric light. What is necessary is either to extinguish the light at intervals, in synchrony with the closing of the lens, or to intercept it so as to keep it off the subject until the exposure is to be made. ”

“ In particular, black body radiation, photoluminescence, generation of cathode rays from ultraviolet light and other phenomena associated with the generation and transformation of light seem better modeled by assuming that the energy of light is distributed discontinuously in space. According to this picture, the energy of a light wave emitted from a point source is not spread continuously over ever larger volumes, but consists of a finite number of energy quanta that are spatially localized at points of space, move without dividing and are absorbed or generated only as a whole. ”

“ These are often designated chemical or actinic rays in order to distinguish the group as a whole from other groups such as ultra-violet, visible, and infra-red. Light is a unique agent in chemical reactions because it is not a material substance. It neither contaminates nor leaves a residue. Although much information pertaining to photochemistry has been available for years, the absence of powerful light-sources emitting so-called chemical rays in large quantities inhibited the practical development of the science of photochemistry. ”

“ Not only do we find that the chemical action is not extended over the whole length of the prismatic spectrum, but we discover that over spaces, which correspond with the maximum points of light and heat, a protective action is exerted. That is, that highly sensitive photographic agents, which blacken rapidly under exposure to diffused daylight, are entirely protected from change in full sunshine, if at the same time as a strong light is thrown upon them by reflection, the yellow and extra red rays are brought to bear upon their surface. ”

“ When white light is decomposed by the prism, we obtain seven groups of visible rays, of unequal refractive power, violet, indigo, blue, green, yellow, orange, and red. The spectrum or ribbon of colors thus obtained widens and spreads out by invisible radiations. Beyond the red, there exist radiations of dark heat, or calorific rays, and, outside of the violet, radiations which are called chemical or ultraviolet rays. The first affect the thermometer, the last occasion energetic reactions in chemical compounds. What is their influence upon vegetation? ”

“ The modifying condition is the quality of light; with reference to this I quote the following from Pfeffer: "So far as is at present known, the action of different rays of the spectrum gives similar curves in regard to heliotropic and phototactic movements, to protoplasmic streaming and movements of the chloroplastids as well as the photonastic movements produced by growth or by changes of turgor. On the other hand, it is the less refrangible rays which are most active in photo-synthesis." ”

“ Some very striking phenomena in photographing the invisible may be produced by the agency of fluorescence. Figures depicted upon a background by one or other of certain substances I shall presently name, although invisible to the eye, may become visible to the camera. Of these, the best known, although not the most effective, is disulphate of quinine. Such a solution, although to the eye it is colourless like water, is to the camera as black as ink. Fill three phials respectively with water, quinine, and common writing ink, and you have two whites and one black ”

“ It would be premature, without further experiments, to speculate too much concerning the nature of these invisible rays; but it is difficult to believe that they can be heat-rays, as the effect is produced through two sheets of hard rubber, containing between them a saturated solution of alum. Although effects are produced, as above shown, by forms of radiant energy which are invisible, we have named the apparatus for the production and reproduction of sound in this way, the "photophone," because an ordinary beam of light contains the rays which are operative. ”

“ Our uncertainty with regard to the correct theory of photographic action is due to great experimental difficulties in studying the problem. As for instance:—
(1.) There is reason to believe that every substance is molecularly affected by radiation, but detection of change is rendered impossible by the imperfections of methods hitherto available, and also by the subsequent self-recovery of the substance in darkness. The effects can be detected in a few cases only when the changes produced happen to be visible, or become visible on development. ”