H. A. van der Velden

Over het aantal lichtquanta dat nodig is voor een lichtprikkel bij het menselijk oog

Summary A method is described for the determination of the number of absorbed quanta in the visual purple necessary for the perception of light in the case of seeing with the rods. From the statistics about the observation of light flashes it is possible to fix this number without knowing anything about the part of the light absorbed by the visual purple. In § 2 the experimental procedure is described. In § 3 a comparison is made between a theoretical relation giving the change for the observation of the light flash as a function of the number of quanta in the flash and the same experimental relation. This theoretical relation (1) is derived on the supposition that k light quanta are necessary for the perception of light. In fig. 2 this theoretical relation is given for the cases k = 1, 2, 3 and 4. From fig. 2 it appears that the case k = 2 agrees with the experimental curve. In § 4 the influence of the duration of the flash is described. In fig. 3 the experimental values of the number of quanta, necessary for an observation chance of 60%, are given as a function of the duration of the flash. In the same fig. 3 a theoretical relation (2) is given which is derived when supposing k = 2 From fig. 3 we can draw the conclusion that the influence of the duration of the flash is in accordance with our conclusion of § 3 that k = 2. In § 5 the influence of the angle at which the spot of light is seen is described. In fig. 4 the experimental values of the number of quanta necessary for a observation chance of 70% are given as a function of the angle under which the light spot is seen. The theoretical relation (4) and (5) for the case of k = 2 is also given in fig. 4. The agreement between theory and experiment is again in accordance with our conclusion that k = 2 from § 3. In § 6 it is shown that it is very probable that the cooperation between the two quanta takes place in the nervous system, concepts which is in accordance with the concepts of nerve physiology.

The Two-Quanta Explanation of the Dependence of the Threshold Values and Visual Acuity on the Visual Angle and the Time of Observation

As shown by van der Velden, for the observation of a short and small light flash it is necessary that at least two quanta of light be effectively absorbed by the visual purple within a time τ (about 0.02 sec.) and within an area corresponding to a visual angle D (about 10′): the two-quanta hypothesis, by which the laws of Ricco, Piper, and Talbot were explained. In the present paper the theoretical foundation is recapitulated and further experiments are described.The extensive experimental results in the present paper are completely covered by the two-quanta hypothesis as far as regards the flash time t<3τ or the visual angle of the light spot d<2D.It is shown, that for the simultaneous occurrence of long flashes and large visual angles, considerable deviations from the theoretical curves derived from the two-quanta hypothesis occur.From these deviations we conclude that T seconds (about 3τ) after the absorption of light quanta the condition of the retina in the neighborhood of these absorptions (within about 3D) is such as to decrease the chance of observation of a subsequent pair of absorbed quanta.Measurements of the visual acuity are found to be in agreement with the two-quanta case.The experimental results of Hecht and Pirenne and the paper by De Vries are discussed, as is also the note of C. Peyrou and H. Piatier about experiments similar to those of van der Velden.

The Two-Quanta Hypothesis as a General Explanation for the Behavior of Threshold Values and Visual Acuity for the Several Receptors of the Human Eye

With the aid of the three methods for the determination of the number of absorbed quanta necessary for light perception described in previous papers, it is found from extensive threshold measurements with flashes for foveal and peripheral vision that a “red cone” in the periphery probably gives rise to a light impression when two quanta are absorbed in it within a time τ.For the foveal cone systems a light impression is caused for every wave-length by the absorption of two quanta within a time τ and within an angular distance of 2–4 minutes. The different kinds of receptors proved to be capable of reacting in mutual dependence on each other, and the conclusion is drawn that all receptors send a nerve impulse to the nerve connection when one quantum is absorbed.A light impression will occur when a second quantum is absorbed after the first absorption within τ sec. within a receptor within a distance D of the first receptor.Experiments on the visual acuity demonstrate that for all wave-lengths, for foveal as well as for peripheral vision, the dependence of the visual acuity on the intensity agrees with the two-quanta theory.

Natural limit of measuring radiation with a bolometer

Abstract In the present article we shall derive the natural precision for the measurement of radiation with a bolometer i.e. the highest precision obtainable with the circuit. In addition we calculate the thermal fluctuation of the bolometer as a function of the frequency, which is of importance when using an alternating current amplifier. It will be shown, that it is possible, when changing the current through the bolometer, to obtain an optimal ratio of signal to noise.

The influence of red and infrared light on a crystal counter

The influence of red light in the reduction of the space charge built up in a crystalcounter, is described, especially for the case of a diamond crystal. The distribution of the pulses and the decreasing of the pulse-magnitude as a function of the irradiation with α-particles, as well as the increasing of this magnitude with red light, is explained qualitatively. It is possible to maintain the counting property of the crystal for an unlimited time, if only the intensity of the light is sufficient.

Differences between counting and non-counting diamonds: Part I: Counting properties and spectral transmission

The luminescence and birefringency of 36 ultraviolet transparent diamonds out of which 13 are counters are investigated. There is no correlation between counting property and luminescence. It is shown, however, that there is a marked difference between birefringency patterns of counting and non-counting diamonds. The patterns of the non counting ones are generally much more pronounced, indicatin g that the counting property is better if fewer cracks are present. Using this as a criterion to eliminate noncounters together with the criterion of transmission at 2536 .A 750,; of the remaining crystals acts as a counter. Moreover it appears from this investigation that laminations in transparent diamonds (diamond II specimens) observed by R a m a n and R e n d a 11 and by R a m a c h a n d r a n are a consequence of the mosaic structure of such diamonds.

Some aspects of the counting properties of diamond

Abstract Experiments carried out on diamond crystal counters are described. The use of a new diamond specimen leads to the same value for the product of the mobility and the mean free time in the crystal as had been found for a previous specimen, the data for which have been already published by the same authors. The energy necessary for the liberation of one electron in a diamond crystal appears to be 8,6 e.V. or less. Evidence is given that the parts of a mixed crystal used for the counting of α-particles are of type II (transparent to ultra violet light).

Photo-electric properties of diamond, measured with a crystal counter

Abstract In this article measurements are described of the function of H e c h t for diamond. The result differs considerably from earlier measurements in which light or electrons are used to irradiate the crystal. Attention is drawn to the important fact that the photo-electric measurements in a crystal, performed with the aid of a crystal counter are more accurate and reliable than older methods.

On some fluctuation problems connected with the counting of impulses produced by a geiger-müller counter or ionisation chamber

Summary In the present article we shall deal with some quantities which are of importance in counting the kicks of a Geigercounter or ionisation chamber arising from some constant or decaying radioactive source. We will first derive the sensibility and the statistical fluctuations of the counting rate meter, using the direct method first indicated by Ornstein and a more classical method for one special case. We also wish to calculate some of these quantities for the case of a scaling set and a mechanical recorder with finite resolving powers. It is shown that in this case the fluctuations are greater (but not much) than in the case of infinite resolving powers.