Licence: Public Domain Mark
Credit: The spectroscope and its applications / by J. Norman Lockyer. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![I] instead of drilling a round hole, used a slit, and to the other additional fact, that Mr. Simms, instead of using that slit with a mere prism, used a lens and made the beani parallel, and then allowed that parallel beam, after it had passed through the prism, to pass into another telescope, and form an image of the slit for each ray. You see how closely con- nected are the grandest discoveries with the skill and suggestiveness of those who supply different instruments for our use. Now I must ask you to come back again to the prism. I have already told you that dispersion is the measure of the difference of the refrangibilities. If we take a prism which appears like an ordinary one, but really is composed of several layers of dif- ferent kinds of glass, and pass an ordinary beam of light through it, it will be differently acted upon by the various layers, and we shall get a difference in the spectra. We have here in fact three distinct spectra, showing that there is something in the different layers of which this prism is composed which turns the light out of its path, and which disperses it more in some cases than it does in others. The cause of this is the density of the glass composing each layer: some kinds of glass are nearly twice as heavy as others, and fortunately we are not limited to glass, for if we were we should not be able to go so far in these inquiries as we do. The prism I am using consists of three separate pieces of glass of different density, and it may be seen that the three st3ectra obtained are differently refracted. It is a very natural conclusion that the heavier and](https://iiif.wellcomecollection.org/image/b21976752_0043.jp2/full/800%2C/0/default.jpg)
