384 



Mr. R. W. Wood on the 



large direct-vision spectroscope. The length of this instru- 

 ment was nearly 2 metres and the dispersion very high. The 

 eyepiece was removed and the slit of a Geneva Society spec- 

 trometer with verniers reading to 10 ;/ was placed in the focal 

 plane. By turning a tangent-screw on the spectroscope the 

 entire spectrum could be made to pass across the slit of the 

 spectrometer. This arrangement permitted the use of approxi- 

 mately monochromatic light of any wave-length desired and 

 of great intensity. The method of observing was as follows : — 

 A reading being taken on the slit, the cyanin prism was 

 introduced and the deviation noted. The cyanin prism was 

 then removed and a glass diffraction-grating put in its place, 

 and the wave-length of the light determined. The tangent- 

 screw was now turned through a fraction of a revolution, and 

 a second observation made ; and in this way about a dozen 

 points were determined on each side of the absorption-band. 

 A second set of observations was then made by opening a 

 small aperture in the black screen near the refracting edge of 

 the prism. By this arrangement the direct image of the slit 

 and the deviated image formed by the prism could be observed 

 at the same time and the distance between them measured by 

 means of a filar-micrometer in the eyepiece. No deviation 

 was produced by the glass plate on which the prism was 

 mounted. The results are given in the following table, and 

 are shown graphically in a curve. 



Cyanin Prism. 



Angle 12' 



35". 



Wave-length. 



Ref. Index. 



Wave-length. 



Eef. Index. 



765 /*/* 



1-93 



497 nn 



1-25 



745 



1-97 



493 



1-29 



723 



202 



484 



1-35 



700 



2-06 



467 



1-42 



685 



2-12 



455 



1-47 



668 



2-19 



4-10 



1-52 



660 



225 



421 



1-55 



648 



235 



410 



1-57 



508 



112 



395 



1-58 



504 



117 







Unfortunately I am obliged to omit the region between 

 X = 510/j//, and A=650yu,/t, since none of these prisms transmit 

 light comprised between these wave-lengths. This is a dis- 

 advantage of course, for this region is the most interesting 



