﻿M. S. Merz on a small Universal Stellar Spectroscope. 1 31 



the field of view the extreme portions of the spectrum, even when 

 extended by the employment of the two systems of prisms. 



With respect to the rectification of the instrument, it is only 

 necessary to observe that all the parts that unscrew are provided 

 with suitable marks ; and, speaking generally, the apparatus may 

 be looked upon as in adjustment when the so-called Fraunhofer 

 lines are situated at right angles to the spectrum. 



The cylindrical lens, it should be remarked, is to be employed 

 only in stellar observations ; in other cases it should be removed. 

 It admits of being turned round in position 90°, and can thus 

 be placed either normal or parallel to the slit. If, for instance, 

 the cylindrical lens is situated within the focus of the refractor 

 and with its axis normal to the direction of the slit, it will elon- 

 gate the little circular disk of a star parallel to the slit, or, in 

 other words, it will form a luminous line coinciding with the 

 direction of the slit, whereas in the opposite case the caustic of 

 the cylindrical lens will lie crosswise to the slit. It should 

 therefore be placed normal to the slit when observing in conver- 

 ging light, or when placed within the focus of the refractor ; 

 while, on the contrary, it has to be set parallel to the slit when 

 observing beyond the focus, or in diverging light, in which case 

 the caustic of the cylindrical lens coincides with the slit as before. 

 And since the rotation of the cylindrical lens causes a change of 

 direction in its caustic to the amount of 90°, this motion admits 

 of the apparatus being always brought into the proper position, 

 whether the cylindrical lens be in the converging or in the diver- 

 ging cone of rays of the refractor. 



Bearing in mind that the little observing- telescope and the 

 collimator are of equal focal length, it is easy to calculate the 

 spectroscopic amplification of the refractor employed. The am- 

 plification is equal to twice that of the focal length of the refrac- 

 tor, expressed in French inches, when the half-inch eyepiece is 

 used, and is equal to the simple focal length when, instead of 

 the half-inch, the 1-inch eyepiece is employed. Were the focal 

 length of the observing-telescope and of the collimator not equal, 

 the amplification would increase as the focal length of the colli- 

 mator diminished, and vice versa. 



With reference to the efficiency of the instrument here de- 

 scribed, I beg leave, in conclusion, to quote the following pas- 

 sage, translated from a letter addressed to me by Dr. Schellen of 

 Cologne, on the 27th of September last. The writer therein says, 

 " after duly adjusting your refractor of 34 lines aperture, in the 

 possession of Herr von Camphausen, I was not only enabled to 

 see at once brightly and with great distinctness the lines H a, H/3, 

 and D 3, but also, on widening the aperture of the slit, to observe 

 at different places on the sun's limb various solar protuberances." 



K2 



