Photographic Method of Registering Absorption-Spectra. 313 



globules from coalescing, and materially promotes the dura- 

 bility of the emulsion. 



(7) Apothecaries' emulsions consist of oil-globules coated 

 with a thin film of gum, which is retained at the oil-surface 

 by molecular force, and prevents the small globules from co- 

 alescing to form drops of oil. 



(8) In the case of castor-oil an emulsion cannot be readily 

 formed, because the soap produced by the contact of the oil 

 with the soda-solution is highly soluble. 



(9) Bile facilitates the solution of the solid soap, and on 

 that account promotes the development of emulsion, in the 

 fluid contents of the intestine, or under certain circumstances 

 may impede it. But the mobility of the oil-surface is increased 

 by the bile. 



(10) From the height of the ascent in capillary tubes, or 

 from the behaviour of fluids at their common surface with air, no 

 conclusions can be drawn as to the phenomena that will occur 

 at their common surface with other liquids or with solid bodies. 



(11) Froth is an emulsion of air instead of one of oil. Its 

 permanence depends on the same physical conditions as that 

 of oil emulsions. 



XLIX. On the Photographic Method of Registering Absorp- 

 tion-Spectra, and its Application to Solar Physics. By 

 Capt. W. de W. Abney, R.E., F.R.S.* 



^i^HBBE are certain difficulties in registering the visible 

 A absorption-spectra as observed, dependent on the eye of 

 the observer, and on his power of representing correctly 

 what he sees ; and it is owing to these deficiencies that 

 curious mistakes have been made in endeavouring to draw 

 absorption-phenomena. Up to the present time it has been, 

 comparatively speaking, useless to attempt such registration 

 by means of photography, owing to the fact that merely one 

 part of the spectrum was impressionable by the silver salts 

 employed as a sensitive medium. Since my discovery that 

 silver bromide could be prepared in such a molecular state as 

 to be sensitive to the whole spectrum (visible, ultra violet, and 

 ultra redf), the difficulty in the employment of photography 

 is done away with ; and it should be taken into use as much 

 as possible, so as to eliminate the errors of eye-observations. 

 A natural objection would arise at first sight, viz. that for the 



* Communicated by the Physical Society. 



t Except those radiations of low amplitude and large wave-length 

 which are emitted by bodies at ordinary temperatures. 



