Luminous Disk Research Articles

Disturbances of rod threshold forced by briefly exposed luminous lines, edges, disks and annuli

1. When the dark-adapted eye is exposed to a brief duration (2 msec) luminous line the resulting threshold disturbance is much sharper (decay constant of ca. 10 min arc) than would be expected in a system which is known to integrate the effects of light quanta over a distance of 1 deg or so.2. When the forcing input is a pair of brief duration parallel luminous lines the threshold disturbance falls off sharply at the outsides of the pattern but on the inside a considerable spread of threshold-raising effects may occur unless the lines are sufficiently far apart.3. The threshold disturbance due to a briefly exposed edge shows an overshoot reminiscent of ;lateral inhibition'.4. If the threshold is measured at the centre of a black disk presented in a briefly lit surround then (a) the dependence of threshold on time interval between test and surround suggests that the threshold elevation is due to a non-optical effect which is not ;metacontrast'; (b) the dependence of threshold on black disk diameter is consistent with the notion that the spatial threshold disturbance is progressively sharpened as the separation of luminous edges increases.5. If the threshold is measured at the centre of briefly exposed luminous disks of various diameters one obtains the same evidence for an ;antagonistic centre-surround' system as that produced by other workers (e.g. Westheimer, 1965) for the steadily light-adapted eye.6. The previous paper (Hallett, 1971) showed that brief illumination of the otherwise dark-adapted eye can rapidly and substantially change the extent of spatial integration. The present paper shows that brief illumination leads to substantial ;inhibitory' effects.7. Earlier approaches are reviewed: (a) the linear system signal/noise theory of the time course of threshold disturbances (Hallett, 1969b) is illustrated by the case of a small subtense flash superimposed on a large oscillatory background; (b) the spatial weighting functions of some other authors are given.8. A possible non-linear model is briefly described: the line weighting function for the receptive field centre is taken to be a single Gaussian, as is customary, but the line weighting function for the inhibitory surround is bimodal.

Nuclear‐Energized Turbidity Monitor for Treatment Plants

The article discusses what is possibly the first application of nuclear energy to water treatment plants in the form of a target for monitoring turbidity after dark. This target is an embedded luminous disc which emits light through the activation of a phosphor by ß‐rays from krypton 85. This device is completely self contained and self sufficient, needing no external current or power pack. Light level is constant enough for purposes of qualitative determination. Radiation hazards are inconsequential, although the proper regulatory agency should be contacted prior to acquisition.

Improvement in Resolving Power of Optical Systems

Image quality is improved definitely by a local variation of the transmission factor which provides in any optical system a non uniform illumination of the exit pupil. One of the most favorable distributions of transmission factor T(X) is the EQUILIBRIUM DISTRIBUTION which corresponds to the greatest concentration of radiant energy in a diffraction pattern around its focus. The criterion used to determine T(X) is the FACTOR OF ENCIRCLED ENERGY E(W) which is the ratio of the energy distributed within a circle of radius W centered on the focus, to the total energy of the diffraction pattern. When an amplitude filter which provides T(W) is fitted to an aberration-free optical system, the diffraction pattern appears, in practice, as a luminous disk without rings of diffracted light. Consequences are in most cases a higher resolution, a more correct contrast relationship and sharper images of extended objects. Moreover geometrical aberrations are partly reduced; For example depth of focus is increased and chromatism partly corrected.

An adaptometer for preparation for fluoroscopy.

Every fluoroscopist has a method of determining when his eyes are dark-adapted; he wears goggles for a timed period, gauges his eyes by a light leak in the room, or perhaps switches the tube on and off until satisfied. Presented here is a low brightness adaptometer which will take the guess out of this chore. It consists of five self-luminous segments which are recessed into a rotating back plate. The luminosity is provided by phosphors excited by the beta radiation from the radioisotope, strontium90,3 and the inserts have the following surface brightnesses: A brighter red fixation point is located just above the window through which the luminous disks are exposed. The shutter is so situated that the fixation point is covered, or partly covered, as desired, when the window is open. The disks exhibit a certain degree of phosphorescence: accordingly the shutter should be kept closed in bright light. If exposure accidentally occurs, a 12-hour period should be allowed before use; otherwise spurious results wi...

THE DIAMETER OF PLUTO

Through the courtesy of Dr. I. S. Bowen and the close cooperation of M. L. Humason, the writer was able to measure the diameter of Pluto with the 200-inch telescope in March 1950. In view of the interest attached to this problem, largely in connection with its bearing on Pluto's mass, the steps leading up to this measurement are recorded below, together with a discussion of some implications of the new result. On repeated occasions, during 1948 and 1949, the writer has attempted to measure the diameter of Pluto with the 82-inch telescope. A disk meter, designed to produce a small artificial luminous disk of controllable brightness, color, and diameter, was placed at the Cassegrain focus. With the aid of small diaphragms the F ratio of the lens forming the artificial image is always made equal to the F ratio of the main telescope. Diffraction effects in the two images to be compared are then identical so that they are eliminated in the comparison. The limits of the instrument are set, of course, by the size of the Airy disk itself, 006 for the 82-inch telescope ; and by obvious intensity requirements (very faint disks, smaller than about 05, look like stars).1 Owing to the faintness of Pluto no reliable results were obtained, in spite of very considerable efforts. The nearest approach to an actual measurement was made on November 4-5, 1949, when the seeing was 9 on a scale of 10 for several hours and the mirror was essentially free from distortions. The writer then derived 0'.'4 for the diameter, and A. Shatzel, who was called in

The flux expansion from the luminous disc.

The flux expansion from the luminous disc.

Radiation from a Perfectly Diffusing Circular Disc (Part I)

An expression is obtained for the amount of flux received from a perfectly diffusing circular luminous disc (i.e., one radiating according to the cosine law) by a second parallel and coaxial disc. This is obtained from Sumpner's theory of the whitened globe, and also from first principles. The following applications of the theory are made: (I.) It is shown that the surface traced out by the edges of coaxial discs which receive equal amounts of flux from a radiating disc is an hyperboloid of revolution about the common axis of the discs. (II.) The amount of flux received from a radiating disc by the walls of a cylinder infinitely long in one direction is found.

On the Function y=J 0 2 (x)+J 1 2 (x) entering in the Diffraction of a Circular Aperture due to a Luminous Disk

On the Function y=J 0 2 (x)+J 1 2 (x) entering in the Diffraction of a Circular Aperture due to a Luminous Disk

The Double Drift Theory of Star Motions

PROF. J. C. CHAMBERLIN'S planetismal hypothesis has given geologists a great deal of matter for thought, and on the whole the phenomena with which they are acquainted appear to fall into line when the earth is considered as a body that has always been solid. The cosmical aspect of the question, which Prof. Chamberlin introduced in advancing his hypothesis, geologists are unable to judge, and they are waiting until astronomers give them an opinion before adopting the hypothesis on the larger scale. On the planetismal hypothesis our stellar system is a disc the edge of which is the Milky Way; beyond lies another stellar system, the so-called nebula in Andromeda for all the most distant stars in the neighbourhood of the nebula appear to be this side of the luminous disc. If our stellar system is of the same nature as that of the nebula in Andromeda, then it must be a spiral nebula with two equivalent arms originating from a central core and winding spirally round the centre in approximately the same plane. Suppose our sun had experienced a gravitational drag and was moving at a less rate than the general average of the other stars, or suppose its spiral course was steeper than the general average, and hence its angular velocity less, then an observer regarding the rest of our stellar system from our planet would see the stars near the centre of the spiral travelling in two directions, those on this side of the centre travelling from right to left, and those on the other side in the reverse direction. Is this not a possible explanation of Prof. J. C. Kapteyn's double-drift theory of star motions? It explains why the two systems travelling in opposite directions should be of equal composition and proportions, but it necessitates that in the region of the sky opposite to that in which the double drift has been observed the drift should be simple.

A Green Sun

I WAS looking, a few days ago, at three o'clock in the afternoon, towards the sun, which was shining in a clear sky. Exhaust-steam from an engine employed in the new Thames Tunnel works, and situated just below my window, was passing intermittently over his face. Many puffs had already crossed it, some partially, others completely obscuring the luminous disk, when presently, three puffs, following each other quickly, successively covered the sun, which then shone brightly through the steam with a vivid light-green colour. The effect was strikingly noticeable, and the green colour intense. I watched for twenty minutes, but in vain, for another “green sun,” and at 3.30 clouds came up.

XIX.—On the Bands formed by the Superposition of Paragenic Spectra produced by the Grooved Surfaces of Glass and Steel. Part II

In the preceding paper I have described the bands produced by gratings or grooved surfaces with 500 divisions in an inch, when the two grooved surfaces are in contact, and the grooves in the one slightly inclined to those in the other.The following results were obtained with two gratings, one of wrhich had 2000 and the other 1000 divisions in an inch.1. When the surfaces are in perfect contact, and the grooves parallel, very irregular bands are seen on the united surfaces, either with a lens or by ordinary vision, and are parallel to the grooves. They are seen only on the 2d, 4th, 6th, &c., spectra on each side of the luminous bar or disc.