DISTINCTNESS OF IMAGE.
The slit must be narrowed until the image is only a fine line, or, in the best thick iron wire dipped into a mixture of salt and hydrochloric acid in the flame of a Bunsen burner. Put the telescope in line with the collimator. The former has an angular motion round the axis support ing the turning table for the prism, and its position is located by a vernier at tached to it, which slides close to a divided circle. Focus the yellow line by adjusting the length of the collimator, and place it on the intersection of the cross wires. The rays coming from are now parallel. Take the reading of the vernier ; this may be called the zero reading. Re place the prism on the turning table, and clamp or fix it with soft wax in the posi tion indicated in the diagram. Find the virtual image of the slit—that is, find the direction of the deviated rays with the eye. Keeping the eye fixed at a distance
from the prism greater than the length of the telescope, bring the latter round to a position in front of the eye. On looking through the eyepiece the slit will again be seen. Rotate the turning table through a small angle, and follow- the image with the cross wires. If the telescope conies nearer the zero reading, continue ; if it spectrometers, a very fine pair of lines side by side. Replace the sodium flame by a batswing burner, or, better still, an oxy-coal-gas limelight, and a continuous spectrum will be seen. An incandescent gas burner would be better than the bats wing. The narrow slit means a loss of light, but it provides a purer spectrum, in which the colours do not overlap. It is impossible, of course, to obviate over lapping entirely.