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to the collar T′T′ which carries this glass, the light proceeding from the second reflexion will be more or less intense, and there will be two particular positions in which there will be no rays reflected at all, of those at least which are regularly reflected by the first glass. Care must be taken to put a dark object behind the glass L′L′ on the side opposite to the reflected light, in order to intercept the extraneous rays which might be sent on this side from exterior objects, and which, passing through the glass, and arriving at the eye, would mix with the reflected rays that are the subject of the observation. The same precaution should be taken for the glass LL; and indeed as this is never used except to reflect light at its first surface, the back of it may be blackened once for all with Indian ink, or smoked over a lamp; it would not do to silver it for a reason that will be given hereafter.
For the light of the candle mentioned above may be substituted that coming from the atmosphere, which may be received into the tube when reflected by the first glass LL; but in this case to preserve to the rays the precise inclination required for the phænomenon, the field of the tube should be limited by some diaphragms with very small apertures placed inside it. The first glass should be blackened or smoked as before mentioned to intercept any rays that might come by refraction from objects situated under it. In this manner, on looking through the tube, when the glass LL is turned towards the sky a small brilliant white speck will be seen, on which all the experiments may be made. The perfect whiteness of this spot is a great advantage; it is an indispensable qualification in many cases, where different tints are to be observed and compared: it is impossible to succeed as well with the flame of a candle or any other inflamed substance, as none of these flames are perfectly white. Lastly, the brightness of the incident light must be modified, so that the portion irregularly reflected by the two glasses may not be sensible; for this portion, being after such reflexion in the state of radiant light, cannot be polarized in one single direction: the other part, which is regularly reflected, alone undergoes polarization, and therefore alone escapes reflexion at the second glass.
Whatever be the nature of the apparatus employed, the process will always be the same, and the same phænomena of reflexion
