of sight is at right angles to the axis of rotation as required. If not, bring the two images into coincidence by first tilting the telescope until their distance apart has been halved, and then tilting the glass plate until the images are in coincidence. Turn the telescope again through two right angles, and if necessary repeat the adjustment till the images coincide with the cross wires in both positions of the telescope.

If the two images of the cross are not both sharply defined, the telescope has not been properly adjusted for parallel rays, or the glass plate is not perfectly plane.

Note.

B

In Fig. 91 let C be the centre of the divided circle. In any position of the telescope let one of the verniers be at A and the other at A', and let O be the axis of rotation of the telescope. Let the telescope be rotated till the verniers are at BB'. Then BOA will be the angle through which the telescope has been rotated, while BCA and B'CA' will be the apparent angular displacements of the two verniers as read off on the divided circle.

B

A

Fig. 91.

Hence by addition BCA + B'CA' = 2BOA

which shews that the angle through which the telescope is rotated, is correctly obtained by taking the arithmetic mean of the angles read off on the vernier scale.

L

SECTION XLII.

DETERMINATION OF THE REFRACTIVE INDEX OF A SOLID BY THE SPECTROMETER.

Apparatus required: Spectrometer in adjustment, glass prism, small mirror and reading lens, sodium bead and flame.

The method used in determining the refractive index of a solid is identical with that explained in the elementary exercise* on the same subject, with which the student is supposed to be familiar. The first step is to determine the refracting angle of the prism. For this purpose the prism must be placed on the table of the spectrometer so that its two refracting faces, and therefore their intersection, are perpendicular to the plane of the graduated circle. The table of

the spectrometer ought to consist of a platform which can be levelled independently of the circular scale. If this is not the case, the prism should be placed on a separate support provided with levelling

screws.

In Fig. 92 PQR are the three

R

Po

Fig. 92.

levelling screws, and the prism is placed in the centre of the table in such a way that one of the faces, say AC, is at right angles to the line joining two screws, say R and Q. This may be done with sufficient accuracy by eye. To adjust the faces of the prism, place the collimator and telescope at a small angle to each other, illuminate the slit and turn the table of the spectrometer until an image of the slit, formed by reflection at AC, appears in the centre of the field.

The slit should be sufficiently narrow to allow you to place the cross accurately over its image; but it is not necessary to have it as narrow as you would use it in spectroscopic work,

* Intermediate Practical Physics, Sec. XXVIII.

unless you desire to resolve the double sodium line and measure the refractive index for each line separately.

The spectrometer table must now be levelled so that the image of a thread stretched across the centre of the slit coincides with the centre of the cross wires of the telescope. The table is then turned till the reflection of the slit is obtained from the face AB, and the image of the thread made to coincide with the centre of the cross wires by levelling the prism by means of the screw P, which is the only one that does not alter the inclination of the face AC, but only turns that face in its own plane. After the face AB has been levelled in this way, return to the face AC; if you have not originally succeeded in placing AC at right angles to QR, you may have slightly altered the inclination of the face. If so, set it right again by the screw Q which least disturbs the face AB.

Going backwards and forwards once or twice will always enable you to secure that the two faces are both properly adjusted.

Proceed next to measure the angle of the prism. Keep the telescope fixed, and turn the prism so that the intersection. of the cross wires lies exactly on the image of the slit formed by reflection at one face of the prism. Read both verniers. In order to see the vernier divisions distinctly reflect light from a gas or electric lamp on to the scale by means of a small piece of mirror glass held near the scale. Next turn the prism till the image of the slip formed by the second face is on the cross wire, and again read both verniers. If the table has been turned through an angle 0° between the two observations, the angle of the prism is 180° – 0°.

If in moving from one position to the other, a vernier passes over the zero of the scale, 360° must be added to the smaller reading, and the higher reading subtracted from it.

Alter the position of the telescope by a few degrees, and repeat your observations, obtaining a second value for the angle of the prism, which should agree closely with the first.

This method implies that the table of the spectrometer is moveable. If it is not so, the method described in the Note p. 207 must be adopted.

Similarly for the second measurement.

Having determined the angle of the prism proceed to find the minimum deviation for light of the kind for which the refractive index of the prism is required, e.g., sodium light. Place a Bunsen burner with a sodium bead in it behind the slit, and without altering the adjustment of the prism turn the spectrometer table so that the light coming from the collimator is refracted through the prism. Find the refracted image of the slit in the first place by eye, and follow the image while the prism is slowly turned round till the position of minimum deviation is found roughly. When this is done bring the telescope into this position, and find the image of the slit in the telescope. Watch this image while the table of the spectrometer is slowly turned. If the direction of rotation is properly chosen, you will find the image moves slowly in the plane of deviation, comes to a standstill, and moves back again. Leave the prism in the position for which the deviation is least, place the cross wires of the telescope approximately on the image, clamp the telescope and adjust the cross wires more accurately by means of the slow motion. Now turn the prism again backwards and forwards, making sure that the centre of the image of the slit just comes up to the centre of the cross wires, but does not pass beyond it. Adjust the telescope if necessary until you are quite satisfied that this is the case, then clamp the table of the spectrometer.

*

* See remarks as to clamping, &c., p. 196.

Now read both verniers. Remove the prism, unclamp the telescope, turn it so as to point directly towards the collimator, and adjust it until the image of the slit is bisected by the cross wires. Read the verniers to determine the position of the telescope when there is no deviation of the ray. The difference in the readings in this position and in the position of minimum deviation gives the angle of minimum deviation. Obtain two independent readings of the position of minimum deviation so as to secure accurate results.

When the spectrometer table is fixed, it is not material whether the direct reading is taken before that of minimum deviation or vice versa. But when the graduated circle moves with the spectrometer table the above order must be adhered to. For it has already been pointed out that correct values will in that case only be obtained if either the telescope only, or the table only, is turned round between two readings. As the adjustment to minimum deviation necessarily involves the turning round of the table, this should be done first and the table be clamped before the telescope is moved round to take the direct reading.

The observations are entered as follows:

Determination of Minimum Deviation.