The perfection of the polarization in a direction perpendicular to the illuminating beam may be also illustrated by the following experiment, which has been executed with many vapours.  A Nicol prism large enough to embrace the entire beam of the electric lamp was placed between the lamp and the experimental tube.  Sending the beam polarized by the Nicol through the tube, I placed myself in front of it, the eyes being on a level with its axis, my assistant occupying a similar position behind the tube.  The short diagonal of the large Nicol was in the first instance vertical, the plane of vibration of the emergent beam being therefore also vertical.  As the light continued to act, a superb blue cloud visible to both my assistant and myself was slowly formed.  But this cloud, so deep and rich when looked at from the positions mentioned, utterly disappeared when looked at vertically downwards, or vertically upwards.  Reflection from the cloud was not possible in these directions.  When the large Nicol was slowly turned round its axis, the eye of the observer being on the level of the beam, and the line of vision perpendicular to it, entire extinction of the light emitted horizontally occurred when the longer diagonal of the large Nicol was vertical.  But a vivid blue cloud was seen when looked at downwards or upwards.  This truly fine experiment, which I should certainly have made without suggestion, was, as a matter of fact, first definitely suggested by a remark addressed to me in a letter by Professor Stokes.

All the phenomena of colour and of polarization observable in the case of skylight are manifested by those actinic clouds; and they exhibit additional phenomena which it would be neither convenient to pursue, nor perhaps possible to detect, in the actual firmament.  They enable us, for example, to follow the polarization from its first appearance on the barely visible blue to its final extinction in the coarser cloud.  These changes, as far as it is now necessary to refer to them, may be thus summed up:—­

1.  The actinic cloud, as long as it continues blue, discharges polarized light in all directions, but the direction of maximum polarization, like that of skylight, is at right angles to the direction of the illuminating beam.

2.  As long as the cloud remains distinctly blue, the light discharged from it at right angles to the illuminating beam is perfectly polarized.  It may be utterly quenched by a Nicol prism, the cloud from which it issues being caused to disappear.  Any deviation from the perpendicular enables a portion of the light to get through the prism.

3.  The direction of vibration of the polarized light is at right angles to the illuminating beam.  Hence a plate of tourmaline, with its axis parallel to the beam, stops the light, and with the axis perpendicular to the beam transmits the light.

4.  A plate of selenite placed between the Nicol and the actinic cloud shows the colours of polarized light; in fact, the cloud itself plays the part of a polarizing Nicol.