Introduction:
In polarimetry, the rotation of plane-polarized light is measured. The angle of rotation can be either positive or negative, depending on the direction of rotation. In this scenario, a reading of 45 degrees has been obtained, and the task is to establish that the rotation is 45 degrees and not -315 degrees.

Method:
To determine whether the rotation is 45 degrees or -315 degrees, the following steps can be taken:

1. Determine the direction of rotation: The direction of rotation can be determined by observing the position of the analyzer. If the analyzer is rotated in the same direction as the polarizer, the rotation is positive, and if it is rotated in the opposite direction, the rotation is negative.

2. Check the position of the polarizer and analyzer: In polarimetry, the polarizer and analyzer are arranged perpendicular to each other. If the analyzer is positioned at 90 degrees to the polarizer, the observed rotation is the maximum.

3. Verify the value of the specific rotation: Each pure liquid has a specific rotation value, which is a constant for that liquid. By comparing the observed rotation with the specific rotation value of the liquid, the direction of rotation can be determined.

4. Check for signs of anomalous dispersion: Anomalous dispersion may occur in some liquids, causing the angle of rotation to be different from what is expected. To check for this, the wavelength of the light used in the polarimeter can be changed.

Conclusion:
By following the above steps, it can be established that the rotation is 45 degrees and not -315 degrees. The direction of rotation can be determined by observing the position of the analyzer, and the specific rotation value of the liquid can be used to verify the observed rotation. If signs of anomalous dispersion are present, the wavelength of the light can be changed to check for any difference in the angle of rotation.