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714 THE PHOTOGRAPHIC NEWS. [November 9, 1883. * Called Fraunhofer lines from their discoverer. a Fig. 40. The lines which would primarily be seen are a" B CD E bFGHK, and their position in the coloured strip, or S3 PF A g E I 3 o P Fig. 41. Fig. 42. will be found that the angle between the green raysG) and the red rays are not equal, nor between any10 i pairs of intermediate rays. This want of equa 1t1t called “ irrationality of dispersion,” and is an imPoires factor in considering achromatism. The above table g examples of this irrationality. P 8 1-332 1-331 1-000 1-334 1-336 1:338 1-341 Water ... 1-527 1-526 1-529 1-533 2-535 1-536 1-542 Crown Glass ... 1-660 1-671 1-628 1-630 1-642 3-723 1-635 Flint Glass . 1-648 G A— All that is necessary for making “ dusting-on pictures ” can be obtained in a polished oak cabinet for a guinea. Ordinary pigments and enamel colours are included. §e with a capital of one penny; the set of apparatus and materials sold for the sum comprising two strips of glass, with a piece of string for binding them together, a few scraps of paper, and a crystal of bichromate of potassium, the whole being contained in a match-box. It is explained by the instructions that a true photograph, let us suppose of a leaf or other opaque object, may be produced with the penny set; but those who wish to do true camera work must obtain the more expensive outfit. The letters below the colours have reference to the fixed Fraunhofer lines of the solar spectrum. Irrationality of dispersion.—Let P and P' be two prisms of different material, and let their repeating angles be so adjusted that rays impinging on each give equal disper sion between the red rays (R) and the violet rays (V), it LESSONS IN OPTICS FOR PHOTOGRAPHERS. BY CAPTAIN W. DE W. ABNEY, R.E., F.R.S. Lesson VII. Chromatic Aberration.—As far as we have gone, it has been assumed that we have been dealing with light of one colour (monochromatic light), and all our conclusions have been based on this assumption. This is not usually the case, and, as a consequence, we may have to modify to some degree our deductions. One of Newton’s early experiments is very well worth repeating in order to satisfy ourselves that a ray of white light does not obey the laws of refraction in such a simple manner as we have assumed. The description of the experiment is from the words of Airy, “ Newton took a black oblong stiff paper terminated by parallel sides, and painted the upper half red and the lower half blue, and viewed it through a prism of a refracting angle of 60° held parallel to the sides of the paper and the cross line. He then found that if the refracting angle of the prism were turned upwards, so that the paper might seem to be lifted up by the refraction, the blue half was lighted higher than the red half ; and if downwards, the blue half was carried lower than the red half; which showed that in both cases the light from the blue half of the paper suffered a greater refraction at the prism than that from the red half.” If, with the same prism, we view a streak of light, such as that coming through the chink between the door jamb and the door, when a window is directly behind it, we shall find that the streak of light is elongated out into a coloured strip. The part most visibly refracted will be seen to be violet; whilst that least refracted will be red. Intermediate between these extremes will be found to be indigo, blue, green, yellow, and orange, these being placed in the order of greatest refraction. One thing must, how ever, be steadily borne in mind, viz., that these colours are arbitrarily named, and that there is no exact place where each of these terminate, but that they shade one into the other. If the chink through the door be narrow, and if the observer stand at some distance from it, it will be found that the coloured strip is traversed vertically by fine black lines ;* and that the narrower the chink, the greater the number of black lines there will appear; but that those first visible are always the strongest. “spectrum,” as we shall in future call it, is fairly indi cated in fig. 40. If a slice of sunlight be examined, the line A will also be seen. The existence of rays beyond H and K can also be demonstrated by their chemical action, and also can be seen by the following artifice. On a piece of very thin microscopic glass, drop a solution of quinine sulphate dissolved in water to which a drop of sul phuric acid has been added, and press another piece.of similar glass on to it, thus forming a thin layer of quinine solution. Hold this to the eye, and examine the spectrum. It will be found that beyond the violet there is a lavender colouration crossed by lines similar to those in the ordinarily visible spectrum, the principal of which are lettered LMN O. Below A, again, the heat ing effect of the spectrum (when it is formed by a strong light, such as direct sunlight, or the electric light), as shown by very delicate thermometers, by the thermopile, or by Langley’s new instrument, the bolometer, demonstrates the existence of rays below A to a distance equal to A G. The following definitions must now be noted. “A ray of white light being decomposed by refrac tion at any surface into a beam of coloured rays, the angle between any coloured ray and the direction of the original white ray is the “ deviation ” of that colour.” The difference of the deviations of two colours is the “ dispersion ” of those colours. The difference between the deviations of the extreme colours is called “ the dispersion of the pencil.'’ It must be remembered that in speaking of a colour, we refer to one of the fixed lines in that colour, and the index of refraction for that colour is designated by placing the letter belonging to that colour below p, Thus the index of refraction of the indigo ray, G, is written u, . [If u, uy be the indices of refraction for the extreme red and violet rays, and u for rays of mean refrangibility out of air into any medium, then P-v — Pt M—1 is called the dispersive power of the medium, and is fre quently denoted by w.]