C. GENERAL PHYSICS. 155 



near the Fraunhofer lines A and B intense black lines are iji- 

 trocluced, while others having but slightly less intensity oc- 

 cur near 48, 11, 81, 99, 101, and 183. Especially remarkable 

 is the great increase of bands in the more refrangible por- 

 tion of the spectrum; thus from 26 to 40, faint bands, and 

 from 46 to 60, from 68 to 74, from 95 to 102, and 107 to 116, 

 very decided bands appear at sunset, which are not visible 

 at midday. Mr. Hennessey has observed sunsets to the 

 number of about 100 since 1870, and has also noticed that 

 during the autumn a kind of haze springs up in the horizon, 

 growing higher and higher from day to day, and denser, 

 forming a permanent winter bank of haze ; by carefully ob- 

 serving the effect of this bank of haze on the solar spectrum, 

 lie has found beyond all question that as this bank rises day 

 by day higher and higher, the corresponding sunset spectrum 

 bands successively disappear, showing that the lines which 

 are visible when the sun sets in the true horizon are intro- 

 duced into the spectrum by reason of the greater thickness 

 of the stratum of air traversed by the sunlight. He states 

 that certain lines, for instance, number 813, are almost as good 

 as a clock. This line commences to change as early as two or 

 three o'clock P.M., and becomes an intense black stripe at the 

 horizon. He states his conviction, also, that besides other 

 changes in the solar light as the sun approaches the horizon, 

 there is this peculiarity that the rays of less refrangibility 

 actually become visible, so that the spectrum apjDcars to be 

 extended toward the right hand. P/iil. 2'rans. Royal Soc. 

 of Zo7i(Io?i, 1815, 157. 



OPTICAL NOTES BY MR. LEA. 



M. Carey Lea, of Philadelphia, states that having a year ago 

 published a train of investigations into the sensitiveness of cer- 

 tain substances to particular rays of light, leading to results 

 incompatible with the late announcement by Dr. Vogel in ref- 

 erence to the sensitiveness of silver bromide when washed with 

 color varnish, he has since then continued his investigations, 

 and while on the one hand Dr. Vogel has adopted his own 

 views, he himself on the other hand has been led to the positive 

 conclusion that there exists no relation whatever between the 

 color of the substances and the color of the ray to whose in- 

 fluence they modify the sensitiveness of the silver bromide. 



