1836.] Measuring Prismatic Spectra. 123 



common light, has been announced, since that of its hetero- 

 geneous nature by the illustrious Newton, with the exception 

 of the syn chronical detection by Wollaston and Fraunhofer, 

 of the constant dark lines which were found in every in- 

 stance, to maintain a fixed and determinate distance from 

 each other. 



The actual measurements and relative extents of the 

 intervening spaces, may thus be considered^ as important 

 data; and any contrivance, however simple, for determining 

 their exact places, will be, it is presumed, acceptable to 

 the practical observer. I therefore propose to describe a 

 very facile method of effecting this purpose, prefixing a 

 brief account of Fraunhofer's telescope for viewing and 

 examining the spectrum. This telescope has a small achro- 

 matic object glass, close before which is placed a short 

 prism, one side of it making a small angle with the axis of 

 the instrument. For viewing the image a positive eye- 

 piece is employed, producing a magnifying power of between 

 twenty and thirty times. In other respects it resembles a 

 small astronomical telescope, having however a much longer 

 range of adjustment, so as to render the image of a near 

 object distinct. Now, the method employed by me of ob- 

 taining the measurement, consists simply in the addition of 

 a circular glass micrometer, placed at the focus of the object 

 glass, it being obvious to every person acquainted with a 

 telescope, that a series of equal divisions placed in the plane 

 of the focus of the eye-glass, will measure the relative dis- 

 tances occurring between the several dark lines in the 

 spectrum, the places of greatest intensity of the different 

 tints, or any other phenomena that may present them- 

 selves. By drawing equi-distant and similar lines upon 

 paper, the image presented by the spectrum may be laid 

 down with the greatest accuracy, or indeed when the 

 colours are sufficiently vivid, they may at once be thrown 

 on the paper by a camera lucida eye piece. 



The micrometers used by me are discs of glass, with from 

 50 to 100 divisions to the inch, and are similar in construc- 

 tion to those employed with my microscopes, except in the 

 omission of the cross lines which are drawn upon the 

 surface of the latter. 



263, Strand, near Temple Bar, June, 1835. 



