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Of the prismatric spectrum in succession, beginning with the red. The rings will then be seen formed successively of each of the colours, perfectly regular in their figure, but contracting rapidly in dimension as they become illuminated with the more refran- gible rays. T At the same time the pole about which they form will be seen to move regularly in the direction of the other axis of the crystal, and if we pass alternately from a red to a violet illumination, will shift its position accordingly, backwards and forwards, through a very considerable angle. If rays of two colours be thrown at once on the apparatus, their two correspond- ing sets of rings will be seen at once, crossing, but not obhterat- ing one another, and the distance between their respective centres will be observed to increase with the difference of their colours. To determine the dispersive power of any medium, and obtain some rough knowledge of its law, we make a prism of it act in opposition to one of a standard substance. prev next |