OPTICS 801 Suppose next that we wish to determine D for the given prism and for sodium light. The slit of the collimator is backed by a sodium flame, the telescope is adjusted for direct vision of the slit, and the reading taken. The prism is now placed upon the table, and rotated until the devia tion of the light from its original direction when seen through the prism is a minimum. The difference of the readings for the two positions of the telescope is the value of D. The angle to be observed may be doubled by using the deviation in both directions. In this case no direct reading in the absence of the prism is required. The following table of indices of refraction is taken from Watt s Dictionary of Chemistry, article " Light." Name of Substance. Index of Refraction. Name of Substance. Index of Refraction. Chromate of lead 2-50 to 2-97 2-47 to 2-75 Phosphoric acid .... Sulphate of copper . . 1-534 1-531 to 1-552 2-224 1-532 2 216 1-527 2 115 Crown glass 1-525 to 1-534 1 95 Nitre 1-514 1-866 1 514 to 1-542 1 Sl to 2 08 1-503 Ruby 1-779 1-500 1-764 Sulphate of potassium 1-500 1-668 Ferrous sulphate .... 1-494 1-610 1-492 Beryl 1-598 Sulphateof magnesium 1-488 1-591 Iceland spar 1-654 1-585 Obsidian 1 488 1-57 to 1 58 Gum 1-476 1-547 1-475 1-545 1 457 1-543 Fluorspar 1-436 1-543 Ice 1-310 Sugar . 1-535 Tabasheer . , 1-1115 to A selection from some results given by Hopkinson, 1 relatin Chance s glasses, may be useful to those engaged in the designing of optical instruments. D is the more refrangible of the pair of sodium lines ; b is the most refrangible of the group of magnesium lines ; (G) is the hydrogen line near G. Hard Crown a. Soft Crown. Extra Light Flint. Light Flint. Dense Flint. Extra Dense Flint. Extra Dense Flint. Specific ) Gravity j 2-48575 2-55035 2-86636 3-20609 3-65865 3-88947 4-42162 B 1 -513625 1-510916 1-536450 1-568558 1-615701 1-642874 1-701060 C .. 1-514568 1-511904 1-537673 1-570011 1-617484 1 644866 1-703478 D .. 1-517114 1-514591 1-541011 1-574015 1-622414 1-650388 1-710201 E .. 1-520331 1-518010 1-545306 1-579223 1-628895 1-657653 1-719114 b .. 1-520967 1-518686 1-546166 1-580271 1-630204 1-659122 1-720924 F .. 1-523139 1-520996 1-549121 1-583886 1-634748 1-664226 1-727237 (G) .. 1-527994 1-526207 1-555863 1-592190 1-645267 1-676111 1-742063 1-528353 1-526595 1-556372 1-592824 1-646068 T677019 1-743204 h 1-530902 1-529359 1-560010 1-597332 1-651840 1-683577 1-751464 HI .. 1-532792 1-531416 1-562760 1-600727 1-656219 1-688369 1-757785 To determine the index of refraction of a liquid it must of course he placed in a hollow prism, whose faces are formed of some trans parent material, usually of glass. The following results of Dale and Gladstone show the influence of temperature upon the refract ing power of some important liquids. They relate to the soda flame, or the line D in the solar spectrum. Tempera ture. Bisulphide of Carbon. Water. Ether. Alcohol Absolute.
1-6442 1-3330 10 1-6346 1-3327 1-3592 1-3658 20 1-6261 1-3320 1-3545 1-3615 30 1-6182 1-3309 1-3495 1-3578 40 1-6103 1-3297 1-3536 50 1-3280 1-3491 60 1-3259 1-3437 Refractive Indices of bisulphide of Carbon for the several Fixed Lines. Temperature. A B D E F G 11 36 -5 1-6142 1-5945 1-6207 1-6004 1-6333 1-6120 1-6465 1-6248 1-6584 1-6362 1-6836 1-6600 Difference 0-0197 0-0203 0-0213 0-0217 0-0222 0-0236 The rapid alteration of refractive power with temperature is a serious obstacle to the use of bisulphide of carbon prisms for exact 1 Proc. Roy. Soc., June 1877. purposes. _ Not only does the dispersive power vary from day to day, but inequalities of temperature in the various parts of the liquid at any one moment disturb the optical uniformity, and are thus the cause of bad definition. A difference of 1 Cent, alters the index about as much as a change in the light from one of the two D lines to the other, so that a variation of one degree within the prism may be expected to prevent the satisfactory resolution of this double line. Excellent results have recently been obtained by Liveing with prisms containing aqueous solution of iodide of potassium and mercury. This liquid can be brought up to a density as high as three times that of water, and gives a powerful dispersion. Some difficulty has, however, been experienced in finding a suitable cement for the faces. Bisulphide of carbon prisms are usually cemented with a mixture of glue and treacle. For many purposes the deviation of the light in passing through an ordinary prism is objectionable. In such cases recourse may be had to direct vision prisms (fig. 12), in which two materials, usually flint and crown, are so com bined that the re fractions are equal Fig. 12. and opposite for a selected ray, while the dispersions are as unequal as may be. The direct vision prism may be contrasted with the achromatic lens (see LIGHT). In the first the object is to obtain dispersion without refraction, and in the second to obtain refraction without dispersion. Compound prisms, composed of a flint between two crowns, are also made, in which the action of the crown is not carried so far as to destroy the deviation due to the flint. By this construction a larger angle is admissible for the more dispersive material, but it is not clear that any sufficient advantage is gained. The principle of the compound prism is carried to its limit by employing media of equal refracting power for the part of the spectrum under examination. For this pur pose bisulphide of carbon and flint glass may be chosen. With Chance s " dense flint " the refractions are the same, and the difference of dispersions is about as great as for "double -extra -dense flint" and crown. A dozen glass prisms of 90 may be cemented in a row on a strip of glass and immersed in a tube of bisulphide of carbon closed at the ends by glass plates. To vary the ray, which passes with out deviation, ether may be mixed with the bisulphide. 2 The formation of a pure spectrum, which may be either thrown upon a screen or photographic plate, or received at once by the eye armed with a magnifier, has been explained under LIGHT. It sometimes happens that the object is not to see the spectrum itself, but to arrange a field of view uniformly illuminated with approximately homogeneous light. For this purpose the pure spectrum is received upon a screen perforated by a narrow slit parallel to the fixed lines. The light which passes this second slit (eye- slit) is approximately homogeneous. Suppose that it corre sponds to the. red of the spectrum. The eye, placed im mediately behind the eye-slit, receives only red light, and, if focused upon the prism, sees a red field of view whose brightness is uniform if the light falling in different direc tions upon the original slit be uniform. To secure the fulfilment of the last condition we may use the light from an overcast sky, or that of the sun reflected from a large surface of white paper. If it be desired to work by arti ficial light, an Argand gas flame diffused by an opal globe will be found suitable. When the adjustments are correct the tint should be perfectly uniform. Any difference _ of colour on the two sides of the field of view is an indication that the screen is not in its proper place. The most important application of this arrangement is to the investigation of compound colours, as carried out by Maxwell. 3 If light be admitted also through a second slit, 2 See "Investigations in Optics," Phil. Mag., January 1880. 3 "Theory of Compound Colours, "Phil. Trans., 1860. XVII. 1 01
