Form of Spectrometer. 77 



{anguishing flame be at a suitable distance, the whole spectrum 

 may be discerned. As that distance is shortened, first the 

 violet and then the other more refrangible colours in their 

 descending order disappear, and at length in the steadily in- 

 creasing effulgence the red alone remains. The yellow never 

 stands out conspicuously, as might have been expected. 



This is scarcely consistent with the assertion that the yellow 

 is the brightest of the rays. The red is plainly perceptible 

 long after the yellow has gone. There is a greenish tint 

 emitted by gas-light that disappears a little previously to the 

 extinction of the red. 



From these observations I think that the luminous intensity 

 of the coloured spaces has a relation to the compression or 

 condensation that the prism is impressing upon them. It 

 may be that, properly considered, the intrinsic intensity of 

 the light is the same for all. In this we must always bear in 

 mind the physiological peculiarities of the eye. 



The foregoing statement is perhaps sufficiently explicit to 

 enable any one to verify the facts. I may, however, mention 

 some improvements in the apparatus, which experience has 

 led me to adopt. 



The intensity of the extinguishing light may be insufficient 

 to obliterate the spectrum even though the slit be closely nar- 

 rowed. How, then, may the intensity of the spectrum be 

 diminished, and that of the extinguishing light be simul- 

 taneously increased ? I accomplished this by depositing on 

 that face of the prism which acts as a reflector an excessively 

 thin film of silver. This, though it was to the transmitted 

 rays quite transparent, increased very greatly by its metallic 

 reflection the extinguishing ones. I could not see any dif- 

 ference between the spectrum of the light that had come 

 through this film and that before the face was silvered, but 

 the reflected light was incomparably more brilliant. The 

 complete obliteration of the entire spectrum presented now no 

 difficulty. 



Nothing need be said about collateral contrivances, which 

 would suggest themselves to any one. A strip of wood a metre 

 long and bearing divisions, served to keep the extinguishing 

 lamp in the proper direction as regards the ground glass, and 

 indicated its distance. I may add, however, that satisfactory 

 observations can be made very conveniently by keeping the 

 extinguishing flame at a constant distance, and varying its 

 intensity by opening or closing the stopcock. This avoids the 

 trouble arising from moving it. In one instrument I caused 

 an index attached to the head of the stopcock to move over a 

 graduated scale, and so ascertained how much it was opened. 



