134 Dr. W. Huggins. On the Limit of Solar and Stelfar 



The original plate has been enlarged about four times; and a 

 spectrum of magnesium and calcium, taken with the same apparatus, 

 and enlarged simultaneously with the plate of stellar spectra, has 

 been placed above to serve as a scale. 



As the spectra are prismatic it is not possible to indicate the wave- 

 lengths in a scale of equal parts. A short scale only is placed over 

 the spectrum where the light of Vega ends. 



The spectroscope with which the spectra were taken is furnished 

 with a prism of Iceland spar and lenses of rock crystal, and a mirror of 

 speculum metal was used to condense the light of Yega upon the slit. 



It will be seen that at my observatory* the light of Vega at about 

 X 3000 is abruptly weakened, and then continues as a very faint line 

 co the point of apparent extinction at X 2970. 



Numerous solar spectra taken here during the last four years with 

 the same spectroscope show an average abrupt weakening at about 

 X 3000, and an apparent total extinction at about X 2985. 



On two occasions only the very faint weakened spectrum could be 

 traced as far as X 2970. 



The abrupt narrowing of the spectrum at the end towards the red 

 is produced by the rapid falling off of sensitiveness of the silver 

 bromide for light of increasing wave-length. 



The increase of breadth of the spectra with increase of duration of 

 exposure is due to the same causes, optical and photographic, which 

 produce the increase of diameter of stellar disks on the photographic 

 plate with longer exposures, when a reflector is used. At h the 

 breadths of the spectra, having 20 minutes and 70 minutes exposure 

 respectively, are 0*06 inch and 0'105 inch."j" 



In 1879 CornuJ made experiments on the limit of the solar spec- 

 trum with reference to the altitude of the place of observation. On 

 the Biff elberg, at an elevation of 8414 feet, the spectrum reached to 

 X 2932, while at the lower elevation of Viege, 2163 feet, the spectrum 

 stopped at 2954. He concludes that the absorption is due to the 

 gaseous constituents, and not to aqueous vapour in the atmosphere. 



In 1881§ Hartley stated that an amount of ozone proportional to 



* Elevation of the observatory 177 feet above mean sea level. Barometer about 

 30"03 inches at the time of observation, 



f The law of increase of size of image with exposure is not as yet accurately denned. 

 Bond found that the diameter of star-disks varied nearly as the square root of the 

 time of exposure. Pritchard, using a reflector, found a law near the fourth root ; 

 and Mr. H. H. Turner has recently found a law very near the cube root for plates 

 taken with a photoheliograph object-glass ( £ Astron. Soc. Month. Not.,' vol. 49, 

 p. 292). 



X " Sur 1' Absorption Atmospherique des Eadiations Ultra-violettes," 'Journ. de 

 Physique,' vol. 10, 1881. 



§ " On the Absorption Spectrum of Ozone, and on the Absorption of Solar Eays 

 by Atmospheric Ozene," 'Chem. Soc. Journ.,' vol. 39, 1881, pp. 57, 111—129. 



