1TJ Instrument for producing Monochromatic L'njht. [Feb. 1, 



their former places, so that their adjustment is unimpaired. A large 

 width of slit, 1 in., is provided for use when imperfectly transparent 

 crystals are under observation whose dispersion of the optic axes for 

 different colours is small, so that a slight curvature of the lines of 

 light vibrating with the same wave-length is immaterial ; but stops 

 of f , , |, and ^ in. respectively are also provided, the two smaller ot 

 which are intended for general nse, and furnish lines apparently 

 perfectly straight. Above and parallel with the fixed circle a second 

 one, which is divided and carries the dispersing apparatus, is capable 

 of rotation. The latter consists of a single 60 prism with truly 

 worked and specially large faces, 4| ins. by 2| ins., in order to fully 

 utilise the light from the 2-in. objective. A single prism is of 

 advantage for the purpose in view, affording more light and the 

 minimum curvature of spectral lines ; in order that the dispersion 

 shall not suffer thereby, the prism is constructed of dense glass, 

 possessing the highest dispersion compatible with perfect freedom 

 from colour, and which will enable the whole of the visible spectrum 

 to be brought between the edges of the exit slit by rotation of the 

 prism without materially sacrificing light by reflection. 



As the optical tubes are exactly similar, either may be chosen as 

 collimator. To a tapped annulus projecting from the slit frame of 

 the one chosen the carrier of an adjustable mirror is attached, and 

 sunlight reflected along the axis of the tube. The other optical tube 

 is then converted into a telescope by the similar attachment of one 

 of three provided eyepieces, which are constructed to focus the edges 

 of the slit immediately in front. The clearly-defined edges thus 

 serve the purpose of a pair of cross-wires between which any solar 

 line may be adjusted. By arranging the prism and telescope so 

 that the beginning of the ultra-violet is adjusted centrally for mini- 

 mum deviation, it is possible by rotation of the prism to bring the 

 whole of the spectrum past the exit slit. The readings of the prism 

 circle are then taken for the positions when prominent solar lines are 

 adjusted between the closely-approximated edges of the slit, and 

 these readings supplemented by those for the red lithium and green 

 thallium lines, and the whole expressed in a table and by a curve. 

 The mirror and eyepiece are then removed. 



Upon illuminating the receiving slit with any artificial source of 

 illumination, light of any wave-length may be made to issue from the 

 exit slit by setting the circle to the reading corresponding to that 

 wave-length. Either the electric arc, limelight, or improved burner 

 and zirconia mantle of the " incandescent gaslight " may be em- 

 ployed, best in a lantern, the condensers of which are sufficient for 

 condensing the rays upon the slit. The opening of the latter need 

 not exceed -^ in. with the feebler of the three sources, and that of the 

 exit slit may be still finer. If either of the two first-mentioned 



