140 
ME. W. HUGGINS ON THE SPECTEA 
former adjusted at the position of minimum deviation for the sodium line D. The train 
of prisms was then enclosed in a case of mahogany, marked a in the diagram, having two 
openings, one for the rays from the collimator b, and the other for their emergence 
after having been refracted by the prisms. These openings are closed with shutters 
when the apparatus is not in use. By this arrangement the prisms have not required 
cleansing from dust, and their adjustments are less liable to derangement. The colli- 
mator l has an achromatic object-glass by Boss of 1-75 inch diameter, and of 10-5 inches 
focal length. The object-glass of the telescope, which is of the same diameter, has a 
focal length of 16-5 inches. The telescope moves along a divided arc of brass, marked 
in the diagram c. The centre of motion of the telescope is nearly under the centre of 
the last face of the last prism. The eyepiece was removed from the telescope, and the 
centre of motion was so adjusted that the image of the illuminated lens of the colli- 
