84 Dr. W. A. Miller on some Cases of Lines 



source employed was, unless otherwise specified, the diffused 

 light of day. The different spectra represented in Plate II. 

 were obtained by I'eceiving the rays upon a prism of Mu- 

 nich flint glass, mounted upon Fraunhofer's plan and ad- 

 justed to an achromatic telescope, placed at 20 feet from a 

 vertical slit in a brass plate, one-fortieth of an inch in width 

 and about 4 inches long. Half of this slit admitted diffused 

 daylight in its ordinary state, the other half of the aperture 

 was covered by the tube or other vessel containing the gas or 

 coloured vapour. The two spectra were thus placed in juxta- 

 position, and the lines of Fraunhofer could, in most cases, be 

 traced across the vapour-spectrum, furnishing exact points of 

 comparison between the two. 



By increasing the depth of the coloured stratum, we in- 

 crease the strength and number of the lines visible; new and 

 fainter lines being rendered discernible by rendering the colour 

 of the gas more intense. A smaller quantity of some bodies 

 is capable of rendering these lines visible than is required to 

 produce the same effect with others; e.g. a quantity of iodine 

 vapour, just sufficient to give a violet tinge to the air in the 

 tube, renders the lines distinctly perceptible, whilst with a 

 tube of bromine, the vapour of which is as full a red as that 

 of nitrous acid, the lines, though visible, are very indistinctly 

 defined. I have, therefore, in experimenting on permanent 

 gases and vapours of liquids volatile at ordinary temperatures, 

 been careful not to decide on the absence of lines unless the 

 light had been transmitted through a body of vapour at least 

 9 inches deep. It is possible that in one or two instances of 

 substances which require high temperatures for their volatili- 

 zation, the employment of a stratum of greater depth might 

 have revealed spectral lines, as, with the exception of sulphur, 

 the experiments were, on account of the difficulty of uniformly 

 heating larger vessels, performed in tubes three-fourths of an 

 inch internal diameter. These exceptions, however, if they 

 exist, will I believe be very few, as in no case, by using thicker 

 strata, have I found lines, when in such tubes all indications of 

 their existence were wanting. The vessels I have found con- 

 venient in operating on the larger quantities of gases are rect- 

 angular boxes of colourless plate-glass, usually cemented with 

 marine glue. 



In connexion with the effect of varying the depth of the 

 coloured stratum, I may mention, that if we increase the in- 

 tensity of the colour, e.g. with nitrous acid by the application 

 of heat, the lines throughout the whole spectrum become 

 much darker, and where they are few in number, more sharply 

 defined ; but where they are numerous, they increase to such 



