The following selection is from: T. Thorne Baker, The Spectroscope and its Uses in General Analytical Chemistry. William Wood and Co. New York (1923).
 

Copyright © 1998 Richard A. Paselk

 

Baker - Table of Contents

 

 

CHAPTER III

 
Adjustment of the spectroscope - Measurement of the refracting angle and the index of refraction of a prism - Measurement of the width of a grating - Refractive indices of liquids - Change of refractive index with change of solvent - Molecular refractive power - Additive nature of atomic refractive powers of elements - Resolving power of prisms and gratings - Absorption by prisms - Anomalous dispersion
 
 
CERTAIN conditions have to be observed in setting tip a spectroscope. The rays of light which emerge from the collimator and fall on the prism face must be parallel; the slit must be parallel to the refracting edge of the prism - in the ordinary way they must both be vertical; where a grating or grating replica is employed as the dispersing medium, the rulings must be perfectly parallel to the slit; the optic axes of collimator and telescope must lie in the same plane.
 
The slit is fixed at the focus of the collimating lens in some instruments, but as a general rule its position is capable of adjustment. In order to adjust the collimator, therefore, the following procedure is carried out: The eye-piece of the telescope is adjusted so that the cross-wires are seen in distinct focus. The telescope is then directed towards some distant tree, church spire, or chimney (the prism being removed), and the eye-piece racked out until it is in distinct focus. The distant object may be looked upon as being infinitely distant, so that parallel rays are incident upon the objective. The telescope is then turned so that it is in a straight line with the collimator, and the slit, which is now illuminated with
 

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46 THE SPECTROSCOPE
 
some convenient light source, should be seen exactly in the centre of the eye-piece, crossing the intersection of the cross-wires or spider-webs. The draw-tube of the collimator is then drawn (or racked) out until the image of the slit is absolutely distinct. The slit should be opened to different widths and examined at each, as this makes it easier to be quite sure when the image is perfectly distinct. Many collimator tubes are provided with a collar and screw, which enable the draw-tube to be fastened quite tightly when once the requisite position has been found. The slit must, of course, be vertical, and its centre must appear at the intersection of the cross-wires. Compensating screws are usually provided, by means of which the telescope and collimator can be corrected if their optic axes do not lie along one straight line.
 
The prism is next placed on the prism table, which frequently has three levelling-screws, by means of which the refracting edge of the prism and the slit can be got into the same plane and perfectly parallel. A certain line, such as the yellow sodium or the green mercury line, should be brought into observation by illuminating the slit with the necessary light and turning the prism table until the ray chosen is at the angle of minimum deviation. The telescope is then very slightly turned in one direction, so that the bright line appears slightly to one side of the centre of the field; the prism is now rotated until the bright line again comes into the centre of the field, and then rotated in the opposite direction until it is again seen there, these two positions being possible, as can readily be seen from an experimental observation. The line should appear equally sharp with the prism in both positions if the rays from the collimator are perfectly parallel; it should also appear absolutely vertical if the refracting edge of the prism has been properly adjusted.
 
 
 

REFRACTIVE INDEX

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MEASUREMENT OF THE REFRACTING ANGLE OF A PRISM. -The prism is placed on the prism table, and the refracting edge is turned towards the collimator. The telescope is now turned in one direction until the image of the slit is seen by reflection from one face of the prism, the slit being centered on the intersection of the cross-wires - i.e., being seen in the exact centre of the eye-piece. The angular deviationof the telescope is then read. The telescope is now turned in the other direction until the slit is again seen owing to reflection from the other prism face; the new angular deviationis read. The difference betweenandgives the required angle. White light may be used in this measurement, which simply depends on reflection, though monochromatic light will give greater precision.