916 
MR. A. E. TUTTON ON AN INSTRUMENT FOR PRODUCING 
aperture, and of filling the similar lens of the exit tube with the dispersed beam. The 
convenience of employing a single prism when rotation is required will be obvious. 
The disadvantage of less dispersion is avoided by constructing the prism of glass 
endowed with as high dispersive power as can be obtained without introducing colour 
and consecj[uent absorption of the violet end of the spectrum. 
4. By placing an eyepiece in front of the exit slit, the optical tube carrying the 
latter may be temporarily converted into a telescope, for the purpose of observing 
solar or metallic lines. The jaws of the exit slit when nearly in contact (their normal 
position when the instrument is being used to produce monochromatic light), are 
clearly focussed by the eye-j^iece and act precisely like a pair of parallel vertical cross¬ 
wires, midway between which any solar or metallic line may be adjusted by suitable 
rotation of the prism. A fine graduation of the circle which carries the latter, aided 
by a vernier, enables this position, for as many lines as it may be desired to observe, 
to be once for all recorded in a table, and graphically expressed by a curve. The col¬ 
limator and telescope remaining fixed, it is only necessary in order at any subsequent 
time to produce light of any desired wave-length to set the prism circle to the reading 
recorded for that wave-length, to remove the eye-piece and to illuminate the slit of the 
collimator by the rays from any source of light whatsoever. The light issuing from the 
exit slit will then be of the wave-length desired. 
5. The narrow band of monochromatic light issuing from the exit slit, when allowed 
to pass directly along the optical axis of the instrument to be illuminated, appears, 
upon looking through the latter, as a brilliant coloured line forming the vertical 
diameter of the field of view. By the simple device of placing a plate of finely ground 
glass immediately in front of the objective of the observing instrument, the line of 
light is diffused so that the whole field of the instrument is evenly and brightly 
illuminated with monochromatic light of the very few wave-lengths which are 
permitted to escape through the exit slit. 
The essential constructive details will now be given. 
Construction of the Instrument. 
The whole arrangement is devised so as to pass as much light as possible, in order 
that when the two slits are almost closed, using the oxy-coal gas lime-light or other 
equally powerful illuminant as source of light, the small fraction of the spectrum 
emerging may still afford ample illumination of the field of the observing instrument, 
after diffusion by the ground glass, to enable accurate observations to be made with 
light as far as G in the bluish-violet, and as far as F when a less powerful illuminant, 
such as an incandescent gas-light burner, is employed. The instrument and its 
various accessories are represented in fig. I. 
The two optical tubes are precisely similar in all respects, so that either may be 
employed as collimator. They are each about nine and a half inches in length, the 
