154 ANNUAL llECORD OF SCIENCE AND INDUSTRY. 



reflecting angle of which is 60 degrees. A slit whose radius 

 is 10 centimeters corrects the distortion of a train of two 

 such prisms. 



SPECTRUM APPARATUS FOR THE NEW OBSERVATORY AT 



POTSDAM. 



The maker of the optical apparatus for the Bothkamp 

 Observatory has lately finished the spectrum apparatus for 

 the new observatory at Potsdam, which probably is the most 

 complete instrument of its kind. The maker, H. Schroeder, 

 says of it that the apparatus consists of 21 single prisms 

 combined into a system according to Rutherford's method, 

 they being moved automatically and in such a way that the 

 motion is accomplished witli mathematical accuracy and 

 'with the greatest ease. This automatic movement allows 

 of exact differential measurements with hitherto unattained 

 accuracy, and is the first apparatus of this kind that has been 

 constructed as an exact instrument for measuring. Accord- 

 ing to Dr. Vogel, the measurements are perfectly trustwor- 

 thy to the one -hundredth part of the interval between the 

 double line D of sodium, the optical 23erformance of this 

 spectroscope being such that the sodium line is separated 

 into nine fine lines. Almost all the principal lines of the 

 spectrum are resolved into groups of lines, while new lines 

 are seen among tliose hitherto known. 7 C, XL, 55. 



THE ATMOSPHERIC LINES OF THE SOLAR SPECTRUM. 



J. B. N. Hennessey, of the Trigonometrical Survey of In- 

 dia, communicates to the Koyal Society of London a short 

 memoir on the atmospheric lines of the solar spectrum as 

 observed by him on the point known as Vincent's Hill, 

 among the Himalaya Mountains, in the northwest province 

 of India, at an altitude above the sea of 1700 feet. His ob- 

 servations wei"e made not only with an old spectroscope, as 

 used in 1868 to 1871, but with a newer and finer instrument 

 supplied by the Royal Society in 1872; and he publishes a 

 map of the solar spectrum, showing the atmospheric lines as 

 observed with the latter instrument. A comparison of the 

 solar spectrum, as seen when the sun was within two hours 

 of the meridian and as seen at sunset, shows the very strik- 

 ing; changes introduced by atmospheric absorption. Thus 



