Another beautiful grotto has green instead of blue refractions; the effect in both cases is due to the light entering by a small entrance.
Although establishing certain general relations between atomic and molecular refractions, the results were somewhat vitiated by the inadequacy of the empirical function which he employed, since it was by no means a constant which depended only on the actual composition of the substance and was independent of its physical condition.
Compounds having the same composition, have equal molecular refractions, and that equal differences in composition are associated with equal differences in refractive power.
Atomic refractions may be obtained either directly, by investigating the various elements, or indirectly, by considering differences in the molecular refractions of related compounds.
Hydroxylic oxygen is obtained by subtracting the molecular refractions of acetic acid and acetaldehyde.
A table of the atomic refractions and dispersions of the principal elements is here given: Dispersion and Composition.-In the preceding section we have seen that substances possess a definite molecular (or atomic) refraction for light of particular wave-length; the difference between the refractions for any two rays is known as the molecular (or atomic) dispersion.
Since molecular refractions are independent of temperature and of the state of aggregation, it follows that molecular dispersions must be also independent of these conditions; and hence quantitative measurements should give an indication as to the chemical composition of substances.
He also showed how changes in constitution effected dispersions to a far greater extent than they did refractions; thus, while the atomic dispersion of carbon is 0.039, the dispersions due to a double and treble linkage is 0.23 and 0.19 respectively.
These are preferably made slightly wedge-shape, to avoid the inconvenience resulting from multiple internal reflections, and they must necessarily be rather thin, so that double refractions due to internal strain may not exert a disturbing influence.
The same conception was utilized by Theodoric of Vriberg, a Dominican, who wrote at some time between 1304 and 1311 a tract entitled De radialibus impressionibus, in which he showed how the primary bow is formed by two refractions and one internal reflection; i.e.
Bartolus, although written some twenty years previously, contains a chapter entitled "Vera iridis tota generatis explicatur," in which it is shown how the primary bow is formed by two refractions and one reflection, and the secondary bow by two refractions and two reflections.
Before 1868 Maxwell conducted the experiment by sending light from the illuminated cross-wires of an observing telescope forward through the object-glass, and through a train of prisms, and then reflecting it back along the same path; any influence of convection would conspire in altering both refractions, but yet no displacement of the image depending on the earth's motion was detected.
If the refractive index is, for instance, the same for both in the case of green light, and a source of white light is viewed through the mixture, the green component will be completely transmitted, while the other colours are more or less scattered by multiple reflections and refractions at the surfaces of the powdered substance.
The fairy women who come to the births of children and foretell their fortunes (Fata, Moerae, ancient Egyptian Hathors, Fees, Dominae Fatales), with their spindles, are refractions of the human "spae-women" (in the Scots term) who attend at birth and derive omens of the child's future from various signs.
Colours are not Qualifications of Light, derived from Refractions, or Reflections of natural Bodies (as 'tis generally believed), but original and connate properties, which in divers Rays are divers.