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SUMMARY1. Membrane currents initiated by intense, 20 lts flashes (photocurrents) were recorded from isolated salamander rods by combined extracellular suction electrodes and intracellular tight-seal electrodes either in current or voltage clamp mode. The magnitudes (mean + 2 S.E.M.) of the maximal photoresponses recorded by the suction and by the intracellular electrode respectively were 40 + 5 pA (n = 18) and 35 + 7 mV (n = 8) for current clamp at zero current; 43±9 pA and 66+13 (n = 11) pA for voltage clamp at the zero-current holding potential, -24+3 mV.2. Photocurrents initiated by flashes isomerizing 01% or more of the outer segment's rhodopsin achieved a saturated velocity and were 95 % complete in less than 50 ms. The effect of incrementing flash intensity above 0-1 % isomerization can be described as a translation of the photocurrent along the time axis towards the origin. Within the interval 0-50 ms the latter two-thirds of the velocity-saturated photocurrent is well described as a single-exponential decay. The decay was much faster in voltage clamp (2-8 + 1P2 ms, n = 11) than in current clamp mode (17 +5 ms, n = 17).3. The initial third of the velocity-saturated photocurrent, occurring over the interval from the flash to the onset of exponential decay, followed about the same time course in current and voltage clamp. The time interval occupied by this initial 'latent' phase decreased with increasing flash intensity and attained an apparent minimum of about 7 ms in response to flashes isomerizing 10 % or more of the rhodopsin at ca. 22 'C.4. The hypothesis that the decay of outer segment light-sensitive membrane current is the same in current and voltage clamp was supported by an analysis of the difference between outer segment currents measured successively in the two recording modes. First, the tail of the difference current decayed exponentially with a time constant approximately equal to R x C, where R and C are independently estimated slope resistance and capacitance of the rod. Secondly, the integral of the difference current, when divided by outer segment capacitance, closely approximated the hyperpolarizing light response measured under current clamp. Thus, displacement current accounted for the difference between photocurrents measured in current and voltage clamp.W. H. COBBS AND E. N. PUGH 5. The hypothesis that a delay stage intrinsic to phototransduction, not voltageclamp resolution, was largely responsible for the 2-8 ms decay rate limitation measured under voltage clamp was supported by two observations. First, clamp settling time was up to 7 times faster than decay of light-sensitive current. Secondly, inner segment membrane current measured with the suction electrode was found to be negligible in a rod that gave a simultaneously measured 45 pA voltage clamp photocurrent with a 2-5 ms decay.6. A cascade of reactions comprising sequential activation of rhodopsin, guanine nucleotide binding protein and phosphodiesterase, depletion of cyclic GMP and cyclic GMP-dependent conductance decrease,...

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A method for following rapid crystallization in polymers using infrared absorption has been devised. The infrared absorption was correlated quantitatively with the density of polyethylene terephthalate for varying degrees of crystallinity and qualitatively with the change in x‐ray diffraction pattern on crystallization of this polymer. This technique has been applied to measure the crystallization–time curves of thin films of polyethylene terephthalate at intervals of 10°C. for temperatures between 120 and 240°C. Half‐times from these curves show a minimum near 190°C. The temperature dependence of the data is discussed in terms of the Volmer‐Becker‐Turnbull theory of nucleation in condensed systems. An activation energy of 20 kcal./mole for short‐range diffusion of chain segments is obtained from the temperature coefficient of the observed induction time. The data on the kinetics of the crystallization are compared with the results of an analysis of Avrami. A constant, A, was introduced into Avrami's equation to give: A is the limiting volume fraction of crystalline material after a long time and was determined from density measurements. The crystallization was found to follow this equation from 2 through 90% transformation for temperatures from 120 to 180°C. Deviation appeared in the region below 10% transformation at higher temperatures. The value of k was interpreted as indicating plate‐like growth at crystallites from 120 to 180°C.

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