Prism (optics)

“ As for the induction, the "three points" are just convenient stopping points on your way down the spectrum. There is a one-to-one mapping of the position of the color in the spectrum coming out of the prism and the index of refraction. This can be proved with the mathematics in the article and the picture of the triangular prism (if you believe the picture of the triangular prism) . Since physical measurements of the wavelengths of optical light can be done in other ways, this sets the condition automatically. If you want to replace the paragraph in the article with a better one, be my guest. ”

“ The Sun-light once broken by a prism does not admit of any farther decomposition of the same kind; for if a portion of the coloured light of any sort be made to pass through a second prism it preserves its peculiar colour unchanged, and the beam of light, has, after refraction, the same form as before. For this reason the rays of any one colour are called homogeneous, and the light simple or homogeneous light.
125. It is remarkable that an object illuminated by homogeneous light has no colour but that of the particular sort of light falling on it ”

“ Perhaps light was so composed that it could not be focused accurately to a sharp and definite point. Perhaps the law of refraction was not quite accurate, but only an approximation. So he bought a prism to try the law. He let in sunlight through a small round hole in a window shutter, inserted the prism in the light, and received the deflected beam on a white screen; turning the prism about till it was deviated as little as possible. The patch on the screen was not a round disk, as it would have been without the prism, but was an elongated [Pg 182] oval and was coloured at its extremities. ”

“ Consequently two prisms thus placed will completely invert the image, producing exactly the same effect as the ordinary inverting system Fig. 106. The simple reversion prism is useful as furnishing a means, when placed over an eye lens, and rotated, of revolving the image on itself, a procedure occasionally convenient, especially in stellar photometry. The two prisms together constitute a true inverting system and have been utilized in that function, but they give a rather small angular field and [Pg 154] have never come into a material amount of use. ”

“ The bigger the objectives the better, provided the prisms are of ample size, while higher power than 6 or 8 is generally unnecessary and disadvantageous. Occasional glasses of magnifying power 12 to 20 or more are to be found but such powers are inconveniently great for an instrument used without support. Do not forget that a first class monocular prism glass is extremely convenient and satisfactory in use, to say nothing of being considerably less in price than the instrument for two eyes. ”

“ The loss of light in the prism glass is really a serious matter, between reflection at the surfaces and absorption in the thick masses of glass necessary in the prisms. If of any size the transmitted light is not much over one-half of that received, very seldom above 60%. If the instrument is properly designed the apparent field is in the neighborhood of 45°, substantially flat and fairly evenly illuminated. Warning should here be given however that many binoculars are on the market in which the field is far from flat and equally far from being uniform. ”

“ For blue and violet it is best to employ the colours of the spectrum. The condensing-lens is then removed, in order that the light may fall directly upon the aperture of the polarizing prism.
The apparatus shown in Plate II. fig. 3. serves to analyse the light by reflexion, and is screwed into the pillar instead of the analysing prism. The screw at holds a concave lens of an equal focal distance. The unbordered mirror inclined at the angle of polarization is 7/4 inch long and 5/4 inch wide. ”

“ Newton’s experiment thus shews that a prism sorts out light of a composite nature according to the wave-length of the different kinds of light present. The same thing can be done by substituting for the prism a so-called diffraction-grating, and this is for many purposes superseding the prism. In general it is necessary, to ensure purity in the spectrum and to make it large enough, to admit light through a narrow slit, and to use certain lenses in combination with one or more prisms or a grating ”

“ What is said of Colours made by Prisms may be easily applied to Colours made by the Glasses of Telescopes or Microscopes, or by the Humours of the Eye. For if the Object-glass of a Telescope be thicker on one side than on the other, or if one half of the Glass, or one half of the Pupil of the Eye be cover'd with any opake substance; the Object-glass, or that part of it or of the Eye which is not cover'd, may be consider'd as a Wedge with crooked Sides, and every Wedge of Glass or other pellucid Substance has the effect of a Prism in refracting the Light which passes through it. ”

“ For larger objectives the prism is usually of decidedly smaller angle, and, if the light warrants high dispersion, several prisms in tandem are used. The objective prism does its best work when applied to true photographic objectives of the portrait lens type which yield a fairly large field. It is by means of big instruments of such sort that the spectra for the magnificent Draper Catalogue have been secured by the Harvard Observatory, mostly at the Arequipa station. ”

“ Mention may also be made of the investigations of Lord Rayleigh and Arthur Schuster on the resolving power of prisms (see Diffraction) , and also of the modern view of the function of the prism in analysing white light. The infra-red and ultra-violet rays are of especial interest since, although not affecting vision after the manner of ordinary light, they possess very remarkable properties. ”

“ When photographs are to be taken the observing telescope is unscrewed and a photographic lens and camera put in its place. If the brightness of the object permits, three prisms are installed, turning the beam 180° into a camera braced to the same frame alongside the slit.
For purely photographic work, too, the objective prism used by Fraunhofer for the earliest observation of stellar spectra is in wide use. It is a prism fitted in front of the objective with its refracting faces making equal angles with the telescope and the region to be observed, respectively. ”

“ Whenever you get this refraction or "breaking back" you also get colour. When I put this prism in the path of a straight beam of light it is bent aside so that it no longer falls on the screen in front, but on this other screen to the side; and at the same time it becomes beautifully coloured (Fig. 26) . The fact is that blue colours are more bendable than yellow, and yellow than red; Fig 27. so that though all the colours start together in the same ray out of the lantern, when the prism bends them, some bend more than others and we get them separated. ”

“ The Lens also ought to be good, such as may serve for optical Uses, and the Prism ought to have a large Angle, suppose of 65 or 70 Degrees, and to be well wrought, being made of Glass free from Bubbles and Veins, with its Sides not a little convex or concave, as usually happens, but truly plane, and its Polish elaborate, as in working Optick-glasses, and not such as is usually wrought with Putty, whereby the edges of the Sand-holes being worn away, there are left all over the Glass a numberless Company of very little convex polite Risings like Waves. ”

“ We can make the light from the top spectrum pass through the coloured body by the following artifice. Let us place a right-angled prism in front of the top slit, reflecting say the beam to the right, and after it has travelled a certain distance, catch it by another right-angled prism, and thus reflect it on to the screen. Already in the path of the ray, issuing through the slit from the bottom spectrum, the lens L4 is placed, forming a square patch on the screen. ”

“ It should be observed that at the surface of bodies the refractive power is exerted, and that the light proceeds in a straight line until leaving the body. The refraction is more or less, and in all cases in proportion as the rays fall more or less obliquely on the refracting surface. It is this law of optics which has given rise to the lenses in our camera tubes, by which means we are enabled to secure a well-delineated representation of any object we choose to picture. ”

“ The weak points of the prism glass are great loss of light through reflection at the usual 10 air-glass surfaces and the general presence of annoying ghosts of bright objects in the field. Most such binoculars have Kellner eyepieces which are peculiarly bad, as we have seen, with respect to reflected images, and present the plane surface of the last prism to the plane front of the field lens. Recently some constructors have utilized the orthoscopic eyepiece, Figure 105a, as a substitute with great advantage in the matter of reflections. ”

“ The diffusing glass placed at an angle has corrected this, so that now the light is evenly distributed throughout. Intensity of light is one of the most difficult things to gauge with the naked eye and a prism glass will invariably give the effect of darkening the room when used in side-light windows, from the fact that the observer cannot look through it. It has been claimed that the intensity of the light admitted through prism glass is the same as that admitted through ordinary glass, but the diffusion is much greater. ”

“ Light is not homogeneous, but decomposable by refraction, absorption, or reflection, into coloured rays of unequal refrangibility. A ray of white light, in passing through a glass prism, is entirely separated into the coloured rays forming the ‘prismatic spectrum,’ and when it passes through a lens, an analogous resolution into coloured rays still occurs, though not so readily observed, and that to an extent often incompatible with distinct vision. ”

“ This is known as [99] chromatic aberration, or the scattering of light in a telescope due simply to its color or wave length. When light passes through a prism, red is refracted the least and violet the most. Through a lens it is the same, because a lens may be regarded as an indefinite system of prisms. The image of a star or planet, then, formed by a single lens cannot be optically perfect ”

“ Prisms.—Prisms are of great value in cases of double vision due to a slight tendency to squinting, caused by weakness or over-action of the muscular apparatus of the eyeball. Prisms deflect rays of light towards their bases. Hence, if a prism is placed in front of the eye with its base towards the nose, a ray of light falling upon it will be bent inwards, and seem to come from a point farther out from the axis of vision. ”

“ Another method largely employed to produce reversed negatives direct, is by means of a mirror or prism placed either before or behind the lens. The prism is the more convenient, but if large sizes are used it becomes a costly piece of apparatus. The mirror, which should be a plane of glass silvered on its surface, is a less expensive affair. ”

“ But it is only from the diffraction, or normal spectrum that any true idea can be gained as to the real distribution of energy among the various constituents, visible and invisible, of a sunbeam. The effect of passage through a prism is to crowd together the red rays very much more than the blue. To this prismatic distortion was owing the establishment of a pseudo-maximum of heat in the infra-red, which disappeared when the natural arrangement by wave-length was allowed free play. ”

“ The inventors claim that mirrors so prepared are able to compete successfully with lenses and totally reflecting prisms—in fact, it was maintained that the silvered glass parabolic mirror possessed the advantages of greater reflecting power and enhanced accuracy, with less divergence of the beam of light.
Owing to the perfection of the lenses and prisms system of lighthouse optics, the introduction of arc lights in conjunction with parabolic mirrors was received with considerable hesitation. ”

“ As a lens may be regarded as built up of a series of thin slices of prisms, divided from each other by planes parallel to the axis of the lens, it will be seen that, if a prism perfectly achromatic for rays of two definite wave-lengths, and approximately achromatic for all rays, can be constructed by combining two prisms of different kinds of glass, all that is required to produce an object-glass with similar small chromatic errors is to combine a convex lens of crown glass and a concave one of flint glass as in fig. ”

“ The horizontal face is fitted with a small cylindrical vessel to hold the liquid. Light is led to the prism at grazing incidence by means of a collimator, and is refracted through the vertical face, the deviation being observed by a telescope rotating about a graduated circle. From this the refractive index is readily calculated if the refractive index of the prism for the light used be known: a fact supplied by the maker. ”

“ From this we say that the sines of the angles of incidence and refraction have a constant proportion or ratio to one another.
The number 1.336 is termed the refractive index, or coefficient, or the refractive power of water. The refractive power varies, however, with other fluids and solids, and a complete table will be found in any good work on optics.
Glass is the substance most commonly used for refracting the rays of light in optical work, the glass being worked up into different forms according to the purpose for which it {128}is intended. Solids formed in this way are termed lenses. ”