266 PHYSICAL SCIENCE 



vapour of great tenuity — the beginnings, perhaps, 

 of suns and worlds yet to be. 



Then came a pause in the progess of this 

 new branch of knowledge. The spectroscope 

 alone seemed to have told all it could to the 

 human eye. A more sensitive instrument was 

 needed to receive its messages, to intensify them, 

 and to interpret them to the senses of mankind. 

 It was not till photography was employed to 

 record the results of spectrum analysis that the 

 full power of the spectroscope was understood. 

 Although previous attempts had been made by 

 means of inferior processes to photograph the 

 spectra of the sun and stars, the great success 

 of the method dates from the application of the 

 dry gelatine process by Sir William Huggins in 

 1876. 



The photographic method has many advan- 

 tages over direct visual observation. The sen- 

 sitive plate can be exposed for a considerable 

 length of time, and the effect of the light on it 

 is cumulative. Excessively feeble light will, by 

 prolonged action, produce a sensible impression 

 on the photographic plate when it would be 

 quite insensible to the eye, which has none of 

 this power of gradually storing and intensifying 

 its impressions. Again, the photograph will 

 record ultra-violet radiation to which the nerves 

 of the eye do not respond, and, in this way, 

 it has revealed many invisible lines. Finally, 

 the photograph forms a permanent record, to 

 which reference can be made at any future 

 time, and permits measurements, more accurate 

 than those made by direct visual observation, 

 to be obtained at leisure in the laboratory many 



