1894.] An Instrument for producing Monochromatic Light. Ill 



IV. " An Instrument of Precision for producing Monochromatic 

 Light of any desired Wave-length, and its Use in the 

 Investigation of the Optical Properties of Crystals." By 

 A. E. TUTTON, Assoc. R.C.S., Demonstrator of Chemistry 

 at the Royal College of Science. South Kensington. Com- 

 municated by Professor THORPE, F.R.S. Received January 

 11, 1894. 



(Abstract.) 



This instrument enables the whole field of any optical instrument 

 whose aperture does not exceed 2 ins. to be evenly and brightly 

 illuminated with monochromatic light of any desired wave-length. 

 It has been devised especially for use in connexion with the axial 

 angle polariscopical goniometers, spectrometers, stauroscopes, micro- 

 scopes, and other instruments employed for the investigation of the 

 optical properties of crystals, but is capable of much more extensive 

 application. It was suggested by the apparatus described by Abney 

 ('Phil. Mag.,' 1885, vol. 20, p. 172), but differs from that arrange- 

 ment in most of its details, and particularly in the employment of a 

 fixed instead of a movable exit slit, of a rotatory instead of a fixed 

 dispersing apparatus, which is capable of accurate graduation for the 

 passage of rays of definite wave-lengths through the exit slit, and in 

 the manner of utilising the issuing line of monochromatic light, 

 which, instead of being directed upon an opaque white screen, is 

 diffused so as to be evenly distributed over the field of the observing 

 instrument when that instrument is placed directly in its path. 



The instrument resembles a compact spectroscope in appearance, 

 and is constructed to pass a large amount of light. Upon a strong 

 stand, furnished with levelling screws, a fixed horizontal circle, 

 carrying a vernier, is supported. About this circle two exactly 

 similar optical tubes are capable of counterpoised rotation ; they 

 carry at the ends nearest the centre of rotation corrected lens combin- 

 ations of 2 ins. aperture and only 9 ins. focal length, and at the 

 other ends a special form of slit, capable of accurate adjustment to the 

 foci of the lenses by rack and pinion movement. The lenses of each 

 combination are not cemented together by balsam, but are held in 

 metal frames, separated by a small air-space, so that they cannot be 

 injured by the heat rays from a powerful source of light, and no 

 alum cell is required. The slit-jaws are capable of equal movement 

 on each side of the central line, so that, however wide the aperture, its 

 centre remains fixed. They may also be removed altogether aud re- 

 placed by a slider carrying two or three slits whenever it is desired to 

 use composite light ; upon replacement they are made to fall exactly into 



