Quantitative laws in biological chemistry
Svante A. Arrhenius · G. Bell eBooks · 1915
What is generally understood by the term " light " is a composite congeries of allied manifestations of energy, comprising such apparently various phenomena as heat, light in the narrower sense of the word, and chemical action.Various as these phenomena are, they are physically identical in character, all consisting of radiant energy in the form of waves of identical character, differing only in the length and rapidity of the vibrations.Broadly speaking, the longest waves cause the sensation of heat, the shortest give rise to chemical action, whilst those of intermediate length cause the sensation of light.If we take ordinary sunlight as the basis of our investigations, it is possible to split it up by appropriate means into its component " rays,"differing from each other in wave-length.Of these certain are visible, and constitute light in the narrower sense of the word, but instead of giving rise to the sensation of white light, they, to the majority of people, show certain pure colours, viz.red, orange, yellow, green, blue, and violet, in order, the red having the longest and the violet the shortest wave-length.The visible spectrum extends from about 723 pp at the red end to 397 ^at the violet end.v. Helmholtz under the most favourable conditions was able to see as far as about 835 /H/A.The limitation of the spectrum at the violet end is less precise, because the rays in this neighbourhood are changed into rays of greater wave-length p. c. v. 1 2 COLOUR VISION by the media of the eye, particularly the lens and retina.This " fluorescence " causes them to produce a lavender-hued sensation, which does not denote true visibility of the short wave-length rays.Beyond the red end are waves of greater length (extending to 60,000 MAi/x.Again, though the ultra-violet rays are particularly potent in inducing chemical action, the visible rays are also, but in less degree, actinic, and the same is true, in still less degree, of the infra-red rays.Further, all rays when absorbed cause a rise in temperature.The most convenient and striking method of demonstrating actinicity is by the photographic film, so that we have come to regard a photograph of the spectrum as a complete analysis of the light under observation, too often forgetting that the photographic effect varies with the specific sensitiveness of the film to particular groups of waves.Thus it is only by specially sensitized films, invented by Sir William Abney, that it is possible to demonstrate infra-red rays photographically.It is further essential that the methods employed for analysis of the light be suitable for their purpose.For example, an ordinary spectro- scope, with glass prisms and lenses, suffices to demonstrate the visible spectrum, but is almost useless for showing the ultra-violet rays, sincethese are absorbed by the glass.In order to demonstrate the full extent of the spectrum it is necessary to use a train of lenses and prisms made of quartz or Iceland spar, which allows a maximum of rays to pass unimpeded.Probably more error has crept into the subject of colour vision from inexact description of experimental conditions and the nature of the stimuli employed than from any other cause.Two green lights may appear identical in colour to the eye, yet their physical characters may differ widely.Again, mixing a yellow and a blue pigment will produce a green pigment, yet the more general statement that green results from mixing yellow and blue is not accurate.The complete range of simple colours can be obtained in a pure state 10 COLOUR VISION of the foveal cones at 0'3 /",, there are about 300 in the long axis, and 60 in the short, 1300 1400 in O'l sq.mm.The diameter of the outer limbs is 0'6 0'75 M. The cones are arranged in curved lines (Max Schultze) or spirals (Fritsch), and are not quite regular.There are small spaces between them, measuring from 0'05 to 0'27 of the trans- verse section of the inner limb.Greeff says that the cones are very closely packed in the fovea, and in a specimen of Heine's were hexagonal in transverse section.Koster 1 examined three normal children's eyes and found that the part completely free from rods occupied a circular area 0'44 0'55 mm. in diameter, the part relatively free from rods 0'88 mm.In the eye of a youth aged 20, the rod-free area was 0'901 mm.He concludes that in the adult the rod-free area measures about 0'8 mm. in diameter, subtending a visual angle of 3 3'.This is probably a maximum, and there is physiological evidence to show that the rod-free area varies in size in different individuals.Three areas must be carefully distinguished : Fovea Centralis, measuring 0'24 0'3 mm. in diameter, subtending 55' 70' ; Rod-free area, measuring 0'8 mm., subtending 3 3' ; Macula, measuring 1 3 mm., subtending 4--12.Dimmer 2 describes a fovea cen trails, 1*5 mm. in diameter (the macula of Koster), containing in its centre a foveola (the fovea centralis of Koster).Gullstrand 3 regards the yellow colouration of the macula lutea as a post-mortem change, a view which is scarcely consistent with its absorptive capacity for coloured lights during life.Fritsch 4 describes the site of clearest vision as the area centralis, possessing a central depression, the fovea centralis, which may or may not contain a foveola.Comparative Anatomy.The distribution of rods and cones in the retinae of lower animals is of great theoretical importance.Many erro- neous statements have gained currency and have been used as arguments in favour of certain theories.The great variety in the forms of the neuroepithelial cells prevents any generalised classification.Green 05 says that there are rods and cones in the retinae of most 1