ASTBONOMY AND METEOBOLOGY. 



NEW SPECTROSCOPE. 



PROF. C. H. YOUNG, of Dartmouth College, thus describes in 

 the "Journal of the Franklin Institute," a new form of spec- 

 troscope: "The instrument was designed for attachment to the 

 equatorial of 6.4 inches aperture and 9 feet focal length, belonging 

 to the observatory of Dartmouth College. It is specially intended 

 for observations upon the solar spots and protuberances, and 

 accordingly the principal object kept in view has been to combine 

 a very high degree of power with compactness, lightness, facility 

 of manipulation, and firmness of construction. Having the dis- 

 persive power of 13 prisms of heavy flint, each with an angle of 

 55, it yet weighs less than 15 pounds, and measures over all 15 

 inches in length, 8 in breadth, and 4 in height. It was made 

 by Alvan Clark & Son. 



" The collimator and observing telescope have each an aperture 

 of seven-eighths of an inch, and a focal length of 7 inches, which 

 might advantageously have been increased to 12 inches, were it 

 not for the necessity of compactness. 



" The light from the slit, after passing the collimator, is transmit- 

 ted through the lower portion of a train of 6 prisms of heavy flint 

 glass, each 2^ inches high, and having, as stated above, a refract- 

 ing angle of 55. A seventh lialf-prism follows, and to the back 

 of this is cemented a right-angled prism, by whish, after 2 total 

 reflections, the light is sent back through the upper part of the 

 same train of prisms, until it reaches the observing-teiescope. 

 This is placed directly above the collimator, and firmly attached 

 to it. Finally, a diagonal eye-piece brings the rays to the eye in 

 a convenient position for observation. 



" The instrument thus has the dispersive power of 13 prisms, and 

 even with the low magnifying power of only 5 on the observing- 

 teiescope, shows perfectly the lines of aqueous vapor which make 

 their appearance between the sodium lines when the sun is near 

 the horizon. Of course, everything shown on the maps of Kirch- 

 lioff and Angstrom is readily seen with it, and many lines besides. 



" Its definition is very beautiful, and the only optical fault of the 

 instrument seems to be"a curvature of the lines, resulting from the 

 shortness of the collimator. 



"After planning the instrument, I learned that the same idea of 

 sending the light twice through the prisms by a right-angled prism 

 at the end of the train had also occurred to Mr. Lockyer and 



305 



