Obtaining the slit image in the field of view the table carrying the prism is slowly rotated. It is observed that the image moving in one direction suddenly reverses its direction of motion. As the prism table is rotated slowly in the same direction, the successive views of the image are observed as in figure. The position of the prism corresponding to the view (c) is the position of minimum deviation.

The setting of the prism at which such reversal occurs is the position of minimum deviation. The telescope is clamped in that position. Next, by working the tangential screw in one direction, it is brought to a position of slightly greater deviation than the minimum deviation position. It makes the prism to rotate so that the angle of incidence is increased by a little. The screw is worked till the image of the slit comes to the edge of the field of view as in position (a), where it appears blurred. The telescope is adjusted to get a fine well defined yellow line. The table is then turned in the opposite direction by working the tangential screw of the telescope in opposite direction so that the angle of incidence now decreases. As the screw is worked the image moves back and again moves forward to come to the same edge of the field of view after going through the minimum deviation position as in (b), Now again at position (b), The image appears blurred.This time the collimator is adjusted to get a sharp well defined image. Thus the telescope and collimator are alternatively adjusted as depicted in figure.

The above procedure of increasing the angle of incidence and adjusting the telescope and then decreasing the angle of incidence and adjusting the collimator is repeated 5 to 6 times till the image appears narrow and sharp in any position. It means that the spectrometer is adjusted for parallel rays. The focussing of collimator and telescope are not disturbed hereafter.