Our knowledge of the mechanism of nerve action is still insufficient to predict what happens in terms of electrical and chemical structure. If our theoretical knowledge were as weak in every aspect as it is upon the behaviour of the excitable membrane, we should not be able to predict confidently any relation but this-that two nerves which were exactly the same in structure, composition and properties, must behave in exactly the same way. However, both Ringer's fluid and axoplasm appear to be homogeneous conductors, so the current distribution in those regions is exactly defined by electrical theory. If, then, two nerves are not identical but have membranes with the same specific properties, it might still be possible to compare them exactly. For, though the properties of the membrane are unknown, they are the same in the two nerves, and though the distribution of current in Ringer and axoplasm is different it is precisely understood.It turns out that there is only a very narrow range of conditions which will permit this argument being applied. It is when the two fibres are dimensionally similar, which is nearly the same as their being geometrically similar. Now the interesting thing is that the nerve fibres do, in fact, exhibit the structural similarity demanded by the theory, and hence it is worth while to examine how far the variation in properties also accords with theoretical predictions. THE ARGUMENTThe assumptions and definitions will first be set out, and then it will be demonstrated that it is only possible to argue from one nerve to another if certain proportions exist between their dimensions. Finally, it will be shown that if these proportions do exist, then the two nerves are dimensionally similar, and that everything which happens in one will, in 'similar ' circumstances, happen to the other with appropriate scaling.

3. By superposing spectral flashes upon steady adapting lights it is possible to find a spectral range in which only one kind of cone is effective. In this range the effect of any spectral light may be matched with that of any other provided the energies are linked in a fixed ratio that defines the action spectrum of the pigment.4. The green pigment has an action spectrum with maximum at 540 nm and corresponds well with the pigment that Marks measured in 'green' cones. The blue pigment has not been measured, but it probably corresponds with that found by Marks in 'blue' cones. However, the red pigment whose action spectrum we measured had its maximum at 680 nm, whereas the difference spectrum of Marks's red cone pigment peaked at 620 nm. The 620 nm cones excite the luminosity S-units but not the R/I units.5. In the range where only one type of cone is effective the relation between the light intensity, Io, and V0, the S-potential generated (both expressed in suitable units), is given by equation (1) p. 545. It is the relation that would be found if cone signals increased the conductance through a polarized 'S-membrane' in proportion to the flux of caught quanta.

SUMMARY1. S-potentials were recorded in fish from units which never responded by depolarization. These hyperpolarizing units are the L-units of Svaetichin & MacNichol (1958).2. Figure 5 shows some sets of action spectra from a single unit. For each curve the criterion of action was hyperpolarization to a fixed level, by lights of various wave-lengths. When these lights fell upon zero background (circles) the curves show that two kinds of cone contribute to the action spectrum, one with the 620 nm pigment of Marks and one with the 680 nm pigment of Naka & Rushton (1966a).3. When the lights fell upon (i) a fixed green background (triangles, Fig. 5), or (ii) a fixed red one (squares), the action spectra changed in a way that indicated greater prominence of (i) the 680 nm system (ii) the 540 nm green system that was not conspicuous without adaptation to red.4. These observations (on the tench Tinca) are contrary to the conclusions of Svaetichin & McNichol (on Gerridae) that the action spectrum is unaltered in shape by adaptation to coloured lights. The contribution of the green cones, for example, was actually absolutely greater under deep red adaptation.5. It is concluded that L-units receive signals from 680, 620, 540 nm and possibly also the blue cones, that the quantum catch in all these contribute to the hyperpolarization produced, but their interaction is more complicated than the simple addition of independent cone effects.

Theoretical equations are derived for the response of a nerve fibre to the sudden application of a weak current. The equations describe the behaviour of the nerve fibre in term s of the membrane resistance and capacity, the axoplasm resistance and the resistance of the external fluid. Expressions are given which allow these four constants to be calculated from experi m ental observations.Axons from Carcinus maenas were used in preliminary experiments. Q uantitative deter m inations were made on a new single-fibre preparation-the 75 y diam eter axon from the walking leg of the lobster (Homarus vulgaris). Currents with a strength of one-third to one-half threshold were used in the quantitative determinations.The behaviour of lobster axons agreed w ith theoretical predictions in the following respects: (a) the steady extrapolar potential declined exponentially with distance; (b) the voltage gradient midway between two distant electrodes was uniform ; (c) the rise and fall of the extrapolar potential a t different distances conformed to the correct theoretical curves.The extrapolar potential disappeared when the axon was treated with a solution of chloroform, indicating th a t the surface membrane was destroyed by this treatm ent, and th a t the potential recorded was in fact derived from the membrane.The ratio of the internal to external resistance per unit length was found to be about 0*7. The absolute m agnitude of the action potential at the surface membrane was estim ated a t about 110 mV.The specific resistance of the axoplasm had an average value of 60Q cm., which was roughly three times th a t of the surrounding sea water.The calculated resistance of one square centim etre of membrane was found to vary from 600 to 7000.Q in thirteen experiments.The membrane capacity was of the order of 1-3/lF cm,-2. No trace of inductive behaviour could be observed in the m ajority of the experiments. B ut three axons with low membrane resistances showed effects which could be attrib u ted either to inductance or to a small local response. The absence of inductive behaviour in axons with high membrane resistance does not prove the absence of an inductive element. Currents w ith a strength several times greater th an threshold often produced oscillating potentials a t the cathode.A local response was always observed when the strength of current approached threshold. The response had a striking inflected form if the current strength was near threshold and its duration less th an the utilization time.Indirect evidence indicates th a t the membrane resistance falls to alow value during activity.on May 11, 2018 http://rspb.royalsocietypublishing.org/ Downloaded from

SUMMARY1. S-potentials from Luminosity-units in the excised eye of the tench (Tinca) were excited by white lights of various intensities and spatial distributions.2. When a small light spot of fixed size and intensity was presented at various distances from the recording electrode, the S-potential was found to suffer an exponential attenuation with distance ( Fig. 3