most difficult part of the measurement consists in finding this distance. It may be determined by comparing it by means of a microscope, with a stage micrometer scale graduated in 01 mm., using a high power objective, then comparing the micrometer scale with a millimetre division on a standard meter scale, using a low power objective. In the present exercise b will be assumed to be known. Record the observations as follows: Mean deviation for first order spectra... 23° 0′52′′. Similarly for the other line, and for the spectra of the 2nd and higher orders. Tabulate results as follows: If the telescope is now turned to one of the first order images, and the table on which the grating is placed is slowly rotated, it will be seen that the position of the image, and therefore the deviation, change. A position of the grating can be found for which the deviation is a minimum. Adjust the telescope on the image when least deviated, and read the verniers. Keeping the table fixed, rotate the telescope till the direct image is seen. Read the verniers. Now rotate table and telescope till the minimum position of the first image on the other side is found, and repeat the observation of the deviated and of the direct image. The minimum deviation & is connected with a and b by the equation 26 sinnλ, hence λ may again be determined. Mean deviation for first order spectra 22° 32′ 15′′. = Similarly for the second order spectra, collecting the results By using the grating so that the diffraction images are formed by rays reflected at the ruled surface, further determinations of λ may be made. S. P. LIBRARY OF THE 15 SECTION XLVII. ROTATION OF PLANE OF POLARISATION. I. Apparatus required: Two Nicol's prisms, two tubes with glass ends, Rochelle salt, a lithium and a sodium flame. When a beam of homogeneous plane polarised light is transmitted through certain solids, liquids, and solutions, it is found that the plane of polarisation is rotated through an angle R proportional to the length of the path of the ray in the substance, to the density d of the substance, and dependent on the nature of the substance and of the light used; i.e. R=d.l. p, where P, ρ is known as the specific rotatory power of the substance for the light used. In general p increases as the wave-length of the light used decreases, hence the necessity for homogeneous light in making measurements. The object of the present section is to verify the laws of rotation of the plane of polarisation of light by solutions of certain substances in a liquid which does not itself produce rotation. In this case the rotation is very nearly proportional to the mass of dissolved substance per c.c. of the solution. The rotation produced by a layer 1 cm. thick of a solution containing 1 gram per c.c. is called the specific rotatory power of the substance dissolved, for the kind of light used. If a solution contains in 1 c.c. a grams of a substance the specific rotatory power of which is p, and a beam of homogeneous plane polarised light passes through 7 cms. of the solution, and R is the rotation of the plane of polarisation produced, then R = a.l.p. Take about 75 c.c. of water, add to it 20 grams of Rochelle salt KNACHO. When the salt has dissolved add water till the volume=100 c.c. If Rochelle salt is not available add 10 grams of sugar to 85 c.c. of water. When the sugar has dissolved dilute the solution till its volume = 100 c.c. If the sugar solution is coloured, mix with it about 2 grams of bone black, allow it to stand a few minutes and then filter. If the filtrate is then bright and clear it may be used. Arrange the solution tube provided, which is a glass tube closed at the ends by moveable plane glass discs, and two mounted Nicol's prisms, in such a way that the light of a Bunsen flame containing a bead of a sodium salt can be seen through the tube and prisms. The Nicol without the circular scale is to be placed between polariser," the flame and one end of the tube, i.e. used as the " and that with the index and circular scale graduated in degrees, placed between the other end of the tube and the eye, i.e. used as the "analyser," Figs. 97 and 98. A thin sheet of yellow glass may be placed between the flame and polariser to cut off the blue light of the flame, and if necessary a lens may be used between the analyser and the eye. Wash out the tube thoroughly, and after filling it with distilled water place it between the prisms and rotate the analyser till no light passes through the system. This will be the case when two lines similarly placed in the Nicol's and at right angles to the line of sight are also at right angles to each other. Read the circular scale on the analyser. Repeat the observation several times, approaching the point of extinction from opposite sides each time, and take a mean of the results. Dry the tube by passing through it a plug of cotton-wool, and then fill it with the salt or sugar solution, place it between the prisms, and determine as before the position of the analyser when no light passes. Repeat, and take the mean. The difference between the two means is the rotation of sodium light produced by the solution. To test the truth of the law expressed by the equation R=a.l.p, take 20 c.c. of the solution, and dilute to 40 c.c. Fill a tube with this solution, and determine the rotation produced. It should be half the amount previously obtained. Wash out another tube, fill with the solution, and place it between the prisms so that the light now passes through the two tubes in succession. Determine the reading for extinction. The rotation produced by both tubes will be found to be double that produced by one, and equal to that produced by one tube of double the strength. Substitute a lithium flame for the sodium flame and determine the rotation produced by a tube of the original solution. It will be found less than that for sodium light. Wash out the tubes thoroughly before putting away the apparatus. Arrange your results as follows: Reading for darkness, with solution of half strength ... ... Reading for darkness, with 2 tubes of solution of half strength B. Lithium Light as above. ... 167.2° ... 156.4° = 10.8° |