New Anali/sis of Solar Light. 15 



of sun-light was thrown in a convenient direction by means of 

 an external reflector. In front, of this hole, but at a distance 

 sufficient to leave room for the insertion of any of my absorb- 

 ing substances, was placed a metallic plate with a narrow slit 

 about two inches long and adjustable in width. The sun-light 

 passing through this slit was received upon a very good prism 

 placed with its axis parallel to the slit, at the distance of about 

 a foot; a large lens was placed in front of the prism, and by 

 means of this the rays diverging from the slit and falling on 

 the prism were made to converge, so as to form upon a paper 

 screen a very good spectrum, sufficiently pure to exhibit se- 

 veral of the stronger Fraunhofer's lines. Care was taken to 

 prevent any other light from falling directly on the screen. 



In order to exhibit the effect of any absorbing medium, it 

 was only necessary to place it behind the slit, in such a man- 

 ner as to receive and transmit the light which was to pass 

 through a portion of the slit. The corresponding portion of 

 the spectrum underwent the modification produced by the 

 absorption : but the modified portion of the spectrum was in 

 no degree dislocated from the unmodified part (Fraunhofer's 

 lines running without dislocation through both), and the modi- 

 fied portion was separated from the unmodified portion only 

 by the small interruption which is usually occasioned by the 

 edges of any refracting substance. It is impossible to conceive 

 circumstances more favourable than these for the comparison 

 of the modified and unmodified spectra. 



Of the absorptive media which I used, I am unable (as I 

 have already stated) to give a complete catalogue. I had 

 however several specimens of sraalt-biue glass, all the varieties 

 of coloured glass which I had been able at different times to 

 procure from a glazier, and all the strikingly coloured fluids 

 which I could obtain from a chemist, as well as port wine, 

 porter, &c. The fluids were used in flat perfume-bottles: it 

 would have been better to use them in slightly prismatic ves- 

 sels, which would admit of the use of difllerent thicknesses of 

 the fluids. 



Of the results of these experiments I can give little more 

 than the single negative one, — that no change was produced 

 in the (]ualities of the colours. After the most patient exami- 

 nation, 1 could not even suspect a change except in two or 

 three instances, when 1 imagined that the blue colour was 

 extended both ways, so as to trench upon the green on one 

 side and upon the violet on the other side : but even these 

 suspicions were removed when I closed with greater care the 

 oj)enings by which a little stray liglit entered between the 

 shutter-hole and the slit in the plate of metal. 



G2 



