98 Prof. Wood on the Dispersion y AJf sorption, 



Dispersion in the Visible Spectrum. 



As the refractive index of the nitroso changes very rapidly 

 with the temperature, it was necessary, in making the deter^ 

 minations of the dispersion, to keep the temperature of the 

 melted substance constant. The point chosen was the solidi- 

 fying point, as it simplified the experimental conditions. The 

 prism was constructed of a pair of interferometer plates ac- 

 curately plane -parallel, and was mounted on the table of a 

 spectrometer in a small clamp-frame made especially for it. 

 A current of hot air was directed against the prism by means 

 of a bent glass tube, under one end of which a small gas- 

 flame was burning. The slit of the spectrometer was illumi- 

 nated with approximately monochromatic light furnished by 

 a monochromatic illuminator built by Fuess. This extremely 

 useful instrument is not as well known as it deserves to be, 

 and a few words regarding it may not be out of place. It is 

 essentially a small spectroscope with collimator and telescope 

 at right angles. The two prisms, which are inclosed in the 

 body of the instrument, are turned by means of a micrometer 

 screw, from the reading of which the wave-length can at 

 once be determined from the calibration-curve of the instru- 

 ment. The eyepiece can be removed, and a draw-tube 

 carrying an adjustable slit inserted in its place. The spectrum 

 can be made to pass across this slit by turning the micrometer 

 screw, and by noting the readings when known lines in the 

 spectrum fall on the slit, the instrument can be calibrated. 

 This is easily and quickly accomplished by means of an in- 

 geniously arranged microscope which can be thrown into and 

 out of position as desired. A small lens carried in a tube in 

 front of the slit focusses the monochromatic light which issues 

 from it on the slit of the spectrometer. 



The dispersion of the nitroso was measured in the following- 

 manner : — The prism having been set at minimum deviation, 

 and the usual adjustments made, the deviated image of the 

 slit, illuminated in monochromatic light, was brought into 

 the field of the telescope. The gas-flame was then moA-ed 

 out of position and the current of hot air stopped. As the 

 fluid nitroso cooled, the deviation increased, the reading being- 

 taken just at the point of solidification, when the image dis- 

 appeared gradually owing to the crystallization of the medium. 

 The warm air w^as then turned on again^ and a second read- 

 ing taken in the same way. A number of prisms were used, 

 the angles varying from 1 to 10 degrees^ those of small 

 angle being necessary when working with the bluish-green, 

 owing to the absorption. In this way the dispersion was 



