376         ON THE  CHANGE  OF  REFRANGIBIHTY  OF   LIGHT.
or two inches, whether parallel or perpendicular to the axis, without any perceptible loss. The contrast between quartz and mica was very striking, for a plate of mica no thicker than paper produced a very sensible diminution in the illumination.
203.    For the  purpose of observing fluids, I procured two vessels consisting of sections of a wide glass tube, about an inch long, closed at one end with a disc of quartz.    I shall call these for brevity quartz vessels, though of course the bottom is the only part in which there is any occasion to use quartz.    When a fluid is to be examined it is poured into a quartz vessel, and then the vessel with its fluid contents is examined in the manner of a solid plate, as described in Art. 201.    On account of the perfect transparency of quartz, the fluid is as good as suspended in air.    When a quartz vessel was partly filled with water, the addition of a very small quantity of nitrate of iron was sufficient to cause the absorption of the active rays.    The solution was so weak as to be almost colourless when  viewed   through   the   thickness through which the rays would have to pass.    A solution of perehlonde of iron had a similar effect.    These fluids I had specially examined by sunlight, and had  not found in them  the least tract; of internal dispersion.    When a fluid exhibits internal dispersion, it is almost always very opaque with regard to rays of high rcfrangi-bility, as is shown, without any special experiment, in the run rue of the observations by which the internal dispersion is exhibited ; but it by no means follows conversely, that when a fluid is very opaque with regard to these rays, though nearly transparent- with regard to the visible rays, it exhibits the phenomenon of internal dispersion.
204.    I  have little doubt that the solar spectrum  would be prolonged, though to what extent I am  unable to say, by using a complete  optical  train   in  every  member  of which   glass  was replaced by quartz.    Such a train would be rather expensive, but would not involve any particular difficulty of execution.    If solid prisms of quartz were used, half of the incident   light would  he lost, on account of the double refraction of the substance, unless the prisms were cut in a particular manner, which however would seem likely to involve some difficulties, both in the execution and in the observations.    But hollow prisms holding fluids might be employed, having the two faces across which  the  light  has to