THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



AUGUST 1875. 



XII. On a New Form of Micrometer for use in Spectroscopic 

 Analysis. By W. M. Watts, D.Sc, Physical- Science Master 

 in the Giggleswick Grammar School*. 



IN determining the position of lines in a spectrum by the use 

 of a micrometer-eyepiece or divided arc, it is often difficult 

 to see the cross-wires distinctly without admitting extraneous 

 light, which with faint spectra frequently cannot be done. I 

 have sought to overcome this difficulty by substituting some one 

 known line of the spectrum itself for the cross- wires, and thus to 

 measure the position of unknown lines, by bringing this index- 

 line successively into coincidence with them. Thus, for example, 

 the sodium-line (which is present in nearly every spectrum 

 whether it is wanted or not) may be made to move along under 

 the spectrum, and the displacement necessary to make it coin- 

 cide with the lines to be measured may be determined by the 

 readings of a micrometer-screw. To accomplish this a convex 

 lens of about 2 feet focus is placed in front of the prism of the 

 spectroscope, between the prism and the observing-telescope, and 

 is divided along a line at right angles to the refracting edge of 

 the prism. One half of the lens is fixed ; the other half is made 

 to slide over it by means of a micrometer-screw. When the 

 movable half of the lens is in its normal position, the only effect 

 is to alter the focus of the telescope slightly ; but when it is 

 made to slide over the fixed half, the refraction of the prism is 

 increased or diminished, half of the spectrum appears to move 

 over the other half, and the sodium-line, or any other con- 



* Read before the Physical Society, June 26, 1875. Communicated 

 by the Society. 



Phil Mag. S. 4. Vol. 50. No. 329. Aug. 1875. G 



