The absorption of light and the colours of natural bodies / by Professor Stokes.

  • Stokes, George Gabriel, 1819-1903.
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: The absorption of light and the colours of natural bodies / by Professor Stokes. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    middle of the hole, a line of blue light for the blue light, a line of red light for the red light, &c., in different positions, so that the spectrum on the screen -would he made up of a series of lines, red, yellow, and blue, &c., if there were so many different kinds of light. Every pure spectrum from a source of light allowed to fall through a slit is to he thought of as made up of a number, generally an infinite number, of images of the slit, corresponding each to the light of one definite refrangibility. Instead of having distinct images of the slit, as you would if there were only a definite number of degrees of refrangibility, if you have all shades you must regard the spectrum as made up of an infinite number of images of the slit placed side by side, and running one into the other with no line of demarcation between them, except in certain conditions in which there is a failure of certain kinds of light. Now the simplest way of seeing this is to take a slit, which may be of the roughest description, and a prism which is just large enough to Cover comfortably the pupil of the eye, and to look at the slit through the prism as I am doing now. I now see a coloured image or spectrum, and in it the fixed lines of Eraunhofer. I presume you have heard of these already, and I shall not describe them, as it would take me too far from my subject. There is one point which I must notice in the use of the prism held in this manner before the naked eye. If you turn it round its axis, you will find that for a certain azimuth of the prism the fixed lines of the spectrum, or at least of a particular part that you are looking at, will be seen distinctly ; hut if you turn the prism a little this way or that round a line parallel to its edge, they will become indistinct. The particular direction or azimuth in which the prism must he held is found by trial. You can focus the spectrum by turning the prism one way or the other, until the image you are looking at is sharp and clear; just as in ordinary cases of focusing. The reason of that is easily explained by geometrical optics, or the science which treats of the mathematical consequences of the laws of reflection and refraction of rays of light. It depends on the alteration of the distance of the virtual focus from which the rays after refraction through the prism come according to the azimuth of the prism. I say virtual focus, although in point of fact, after passing in that manner through a prism, unless it be in the position of minimum deviation, the light diverges, not from