Light from a Dense to a Rarer Medium. 351 



diffraction spectrum to be seen in a direction corresponding 

 to = 90°, a grating of more than 2000 lines to the inch is 

 required. 



We have been able to realize experimentally the results 

 deduced from theory above. We used a reproduction of a 

 Robert's grating, of 3000 lines to the inch, photographed on 

 a very thin collodion film, and afterwards intensified with 

 mercury. The clear side of the glass carrying the grating was 

 cemented with Canada balsam to the hypotenuse face of a right- 

 angled prism, of which the remaining angles were each equal 

 to 45°. The lines of the grating were adjusted to be perpendi- 

 cular to the triangular ends of the prism. The whole was then 

 mounted on the central table of a spectrometer, one of the 

 mutually rectangular faces of the prism being normal to the 

 parallel beam from the collimator, while the lines of the 

 grating were vertical. Brilliant diffraction spectra were seen 

 on looking at the orating surface, either with the unaided 

 eye or through a telescope f ocussed on infinity. The brightest 

 spectrum was that seen on looking along the surface (6 nearly 

 equal to 90°). In directions corresponding to smaller values 

 of 6 spectra of higher orders were seen ; the brightness of 

 these spectra decreased as the order increased, but they only 

 became invisible at an angle of diffraction equal to 10° or 15 5 . 

 On rotating the prism and grating through a small angle the 

 central undifTracted image was brought into view ; this, of 

 course, took the form of a prismatic spectrum, the light having 

 been refracted at the prismatic faces inclined at an angle of 

 45°. The central image was easily recognized owing to the 

 circumstance that the blue ends of the diffraction spectra on 

 either side of it were turned towards each other. On rotating 

 the prism so as to increase the angle of internal incidence on 

 the grating surface the central image disappeared, but no 

 change occurred in the general appearance of the diffraction 

 spectra. On continuing to rotate the prism in the same 

 direction one after another of the diffraction spectra dis- 

 appeared, but light still emerged when the prism had been 

 rotated through 25° or 30°. A careful examination left no 

 doubt in our minds that the spectra were formed in accordance 

 with the theory sketched out above. 



Of course in the present case the light escaped into the 

 air from the surface of the collodion film. But it is easily 

 proved that when the light is incident on the glass-collodion 

 interface at an angle exceeding the critical angle for glass- 

 air, the light refracted into the collodion must fall on the 

 collodion-air surface at an angle exceeding the critical value 

 for collodion-air. The refractive index of Canada balsam is 



