﻿THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



OCTOBER 1894. 



XXXVIII. Fixed- Arm Spectroscopes. 



By Frank L. 0. Wadsworth*. 



[Plates VIII.-X.] 



IN prismatic spectroscopes and spectrometers of the usual 

 construction it is necessary, in order to observe different 

 portions of the spectrum under the same conditions, to vary 

 the angle between the axes of the observing and collimating 

 telescopes by the rotation of the arm which carries one of 

 these telescopes about an axis parallel to the refracting edge 

 of the prism. Usually the arm which carries the slit and the 

 eollimating-lens is fixed, and that carrying the observing 

 telescope is movable ; but sometimes the apparatus which it 

 is necessary to carry on the observing arm is so massive, or 

 else requires such a degree of stability, that it becomes neces- 

 sary to fix it in position and make the slit-arm the movable 

 one. Then difficulties are at once encountered in the illu- 

 mination of the slit, if a fixed source of light such as the sun 

 or a star is under examination. Even if terrestrial sources 

 be employed, it is ofttimes extremely undesirable to have a 

 complicated system of collimator, slit, condenser, and source of 

 light, swinging about on a long arm, and in particular cases 

 such an arrangement is absolutely inadmissible. In such cases 

 where both arms of the spectroscope are necessarily fixed, we 



* Communicated by the Author. 

 Phil Mag. S. 5. Vol. 38. No. 233. Oct. 1894. 2 A 



