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rays, light, red, spectrum, prism, lines, violet, slit, eye and visible

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SPECIFIC HEAT.—A term applied to the quantity of heat required to raise equal weights of different substances through equal intervals of temperature. Water is taken as the standard substance in measuring quantities of heat.

SPECTROSCOPE.—An instrument employed in spectrum analysis. It usually consists of the following parts: (I.) A tube with a narrow slit at one end and a convex lens at the other, from which parallel rays of light pro ceed when light is made to pass through the slit, the two forming together what is termed the collimator. (2.) A prism of dense flint glass on which the rays fall after emerging from the collimator. (3.) An observing telescope so placed that the rays traverse it after emerging from the prism.

Fig. 423 gives a ground plan of the arrangement: S is the slit, C the collimating lens, P the prism, 0 the object glass of the telescope, and E the eye-piece. An image of the slit will be formed at f by rays of given refrangibility, others between and v by rays of greater refran gibility, and others between f and r by rays of less refrangibility, thus giving a complete spec trum.

Burton in his " Photographic Optics " says : " Sometimes quite a number of prisms are used to gain greatly increased dispersion. An arrangement—the invention of the famous optician, Browning—for the use of six prisms, is shown in Fig. 422: the dotted line shows the direction taken by a ray of monochromatic light. Mention must on no account be omitted of the direct vision spectroscope, as this is a little instrument particularly useful to photographers.

By referring to what has been said in Chapter III* on the means of correcting chromatic aberration, and considering how we may bend a ray of light without dispersing it, it will readily be understood that, by a modification of the same arrangement, we may disperse a ray of light without bending it. By so doing, we may have a spectroscope with the collimating tube and the eye-piece in a straight line, in place of at an angle with each other. Only a limited amount of dispersion is, of course, permissible with this arrangement, or the spectrum would pass beyond the field of view.

The next two cuts, Figs. 42o and 421, show the arrangement of prisms for dispersing a beam of light without deflecting it. In each case the darker shaded prisms are of flint, the lighter of crown glass.

The chief use of the direct vision spectroscope to photographers is in examination of mediums for dark-room illumination. Most photographic work is done by light of short wave lengths—blue, violet, etc. Sensitive films are, therefore, most easily worked in light of long wave lengths – red, yellow, etc. Colors that appear to the eye quite simple are often in reality compound. Thus, there are two kinds of red glass, to the eye as like each other as can be, but one of which transmits only red light and, perhaps, a very little green, while the other transmits a considerable amount of blue or purple also. The latter, it will be evident, is quite unsuited to dark-room illumination. The two cannot, as has been said, be distinguished by the eye, but they can at once when the light passing through them is examined by the spectroscope.

The spectrum extends in both directions beyond what can be seen by the eye, the rays of greater and less refrangibility than those visible to the eye being termed ultra-violet and ultra red rays. Photography has been particularly useful in mapping these.

There is the objection to the prism spectroscope that the glass of the prism absorbs—or stops—some of the invisible rays. Fortunately a spectrum can be obtained otherwise than by a prism. It can be got by the aid of a ' diffraction grating.' A diffraction grating is a polished surface—such as speculum metal—ruled with lines so close together that there are many thousands to the inch. This, on account of what is called interference of light, gives a very perfect spec trum, none of which is absorbed. The subject of the diffraction grating is rather too complex to treat of in detail here." stripe formed on a screen by a beam of light, as of the sun, received through a narrow slit and passed through a prism, being thus decomposed or separated into its constituent rays. This oblong stripe consists of a number of colors shading imperceptibly into one another from red at one end, through orange, yellow, green, blue, indigo, to violet at the other. (See Fig. 424.) These colors are due to the different constituents of which solar light is made up, and the stripe seen is formed by an indefinite number of images of the slit ranged in order and partially over-lapping. The analysis or decomposition of the beam is due to the different refrangibilities of the component rays, the violet being the most refrangible and the red the least. Besides the visible color rays, the spectrum contains thermal or heating rays, and chemical or actinic rays, which are not visible to the eye. The heating effect of the solar spectrum increases in going from the violet to the red, and still continues to increase for a certain distance beyond the visible spectrum at the red end, while the chemical action is very faint in the red, strong in the blue and violet, and sensible to a con siderable distance beyond the violet end. The actinic rays beyond the violet maybe rendered visible by throw ing them upon a surface treated with some fluorescent substance.t Besides this band of different colors, a pure solar spectrum will be found to possess a number of dark lines, which cut through it perpendicularly. These lines were first discovered by Wollaston, and were carefully studied by Frauenhofer and called after him Frauenhofer's lines. The lines are always to be found on the same spot, and serve to indicate the exact places on the scale of color. For instance, if we use the term blue in the spectrum, this is naturally vague and incomplete, but by indicating the line on the spectrum in which this color is found no difficulty will arise. For this purpose Frauenhofer gave certain characteristic names to the line, which he indicated by letters. A certain line in the red he called A, another in the yellow D, and so on. As, however, these lines number some thousands it is evident that these letters will not suffice to indicate them all. Fig. 424 shows the position of the - - - - - most conspicuous of these fixed lines, and the letters above them are the names by which they are known.

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