Measurement and implications of Saturn’s gravity field and ring mass

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn’s gravity deviates from theoretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ± 0.49) × 1019 kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 107 to 108 years ago.

Macular sensitivity in patients with congenital stationary night-blindness

AimTo evaluate and correlate mean light sensitivity thresholds (MLST) in patients with congenital stationary night-blindness (CSNB) in comparison with healthy subjects using microperimetry (MP1).MethodsEleven patients with CSNB and 13 healthy subjects were compared. In all subjects, static threshold perimetry was performed using MP1 evaluating the central 6 mm of the retina. This central retinal area was divided into three rings through using the ETDRS grid algorithm with an innermost (1 mm), inner (3 mm) and outer ring (6 mm). The MLSTs were acquired in nine sectors of the ETDRS grid. A comparison of MLST was performed between both groups using a t-test (significance level p<0.005).ResultsA significant reduction of MLST in the fovea (innermost ring, 1 mm) was observed for patients with CSNB (7.2±3.90 dB) in comparison to healthy subjects (19.7±0.75; p<0.0001). Similarly, comparison of MLST in all other sectors (superior/inferior/temporal and nasal) within the inner and outer ring revealed a statistically significant reduction in patients with CSNB compared with healthy subjects (p<0.001).ConclusionsExamination of macular retinal sensitivity intensity using MP1 revealed for the first time a significant reduction of MLST within the central 6 mm of the retina in patients with CSNB compared with healthy subjects. This finding supports MP1 as an additional diagnostic tool when examining patients with retinal dysfunctions such as CSNB.

The development of spatial patterns

It is convenient to start with an account of the development of the flower in the crucifer Arabidopsis (Coen & Meyerowitz, 1991), for several reasons. The development of plants is in one respect simpler than that of animals: cells do not move relative to one another. Much of animal development is achieved by cell movement, contraction and adhesion; these are processes we need not bother with when thinking about plants. As yet, less is known of Arabidopsis than of Drosophila or of vertebrates: what is known is simple and elegant, and brings into sharp focus the things that are not yet known. The flower develops from a disc of cells. Four concentric rings differentiate. The outermost ring gives rise to four sepals, the next innermost ring to four petals, the next to six stamens, and the central cells to two fused carpels. Mutants are known that alter this pattern. Usually, a given mutant alters two neighbouring rings of organs. A simple model explains these mutants. Genes are expressed in three regions, A, B and C. If we symbolize the corresponding genes by a, b and c, then a gives rise to sepals, ab to petals, be to stamens and c to carpels. This would account for normal development, but if we are also to explain the mutants, we must assume that genes a and c are mutually inhibitory, so that when a is absent or deficient, c is expressed over the whole region, and vice versa. This model has been confirmed in two ways. First, it can satisfactorily explain double mutants. Triple mutants, in which all three gene activities are missing, develop leaf-like structures in all four rings: this confirms the longheld view that flower organs are modified leaves. The phenotypes of the various double mutants can be predicted from the model, and are as expected. The model has been confirmed more directly by hybridization in situ of the RNA transcripts of the different genes, which appear early in the development of the flower primordium in the expected places. An almost identical system is present in Antirrhinum. There is molecular homology between the genes in the two species.

On the optical phenomena of certain crystals

In this memoir the author gives an account of the optical properties of certain minute crystals, obtained by the evaporation of a solution of borax in phosphoric acid, exhibited when they are examined by means of the polarizing microscope. The field of view is then seen covered with minute circular spots, each composed of a close assemblage of delicate acicular crystals, radiating from the centre ; together with other circular bodies, in which this disposition is not observable, on account of the close union of the component crystals, which, producing optical contact, gives perfect transparency to the whole mass. When the field of view is rendered dark by the rectangular crossing of the polarizing laminæ, each of these little circles becomes luminous, and exhibits a well-defined dark cross, dividing its area into four equal sectors. These crosses have a similar position in all the circles ; and their direction remains unaltered when the crystals are turned round in their own plane, by causing the plate of glass, on which they are placed, to revolve. On examining the larger circles with a high magnifying power, and under favourable circumstances of illumination, the author observed upon each a series of coloured concentric rings : but the number as well as the colour of these rings varies in different crystals. The innermost ring is deeply coloured, or black ; and incloses a central space of white light, which is traversed by the arms of the cross, intersecting in the centre. This part of the cross, which stands within the innermost ring, is beautifully defined, and perfectly black. The whole system of phenomena are exactly analogous to that exhibited by uniaxal crystals ; and corresponds still more closely with those discovered by Sir David Brewster in spheres of glass, the density of which had been rendered variable from the centre to the surface by immersion in heated oil ; excepting that the miscroscopic crystals here described are possessed of a far more intense polarizing energy. The author thinks it probable that the phenomena are in both cases produced by similar conditions of density ; which, in a circular mass formed by the aggregation of needle-shaped crystals radiating from a common centre, it is natural to suppose would rapidly increase from the circumference to the centre. By watching the progress of crystallization he ascertained that this was, in fact, the mode in which the crystals are constructed ; for they frequently appeared, at first, in the form of lengthened prisms, which subdivided themselves at both ends into an immense multitude of divergent fibres, like those of a brush ; apparently repelling each other as they extended in length, and occupying spaces corresponding to two opposite sectors of a circle ; until, by spreading still farther in breadth, their edges united, and filled the whole of a circular area. In all the stages of this process, the formation of the black cross may be seen to keep pace with the developement of the crystal, until perfectly displayed on the completion of the crystalline structure. The author notices the analogy which this structure presents with that of the crystalline lens of the cod fish, as has been lately described by Sir David Brewster ; and also the remarkable correspondence existing between the optical properties resulting from this structure, and the phenomena of the circular polarization of fluids, which have been accounted for by the existence of molecules of a structure nearly similar.