of Light and its Theory. 259 



place with sufficient brevity, and without diagrams. With 

 some systems of lines of this kind several spectra or parts of 

 them may be wholly wanting, or have so slight an intensity 

 that they are not easily observed, whilst the succeeding ones 

 again become very intense. This affords the great advantage 

 that the fixed lines of these spectra may be observed; in 

 systems of lines, consisting of equal spaces s C^^^ and F^" 

 can be seen ; with a regularly unequal system of lines, how-i 

 ever, where every division consists of three shades g, different 

 among themselves, and are as 25 : 33 : 42 ; Cxn, D^", E^", 

 and F^" are so distinctly seen that their distances from the 

 axis can be measured with certainty. For with such systems 

 of hues the tenth and the eleventh spectra are almost wholly 

 wanting. With this system of lines I myself saw E^xiv gtiH 

 so distinctly, that its distance from the axis could be measured. 

 The proportion of intensity of the different spectra depends 

 on the proportion of the spaces, as they follow each other in 

 one division. This in many cases is very complicated. * Si- 

 milar regularly unequal systems of lines are obtained when 

 two different systems of lines, that is, two in each of which 

 the spaces s are equal to each other, but in one larger than 

 in the other, are so placed with the etched surfaces together 

 that the lines shall run exactly parallel. 



It cannot be uninteresting to produce perfect spectra of 

 the second class which form concentric circles, and in which 

 the fixed lines consequently appear circular. From what 

 is already known, it may be easily inferred that such spec- 

 tra must be produced when the system of lines consists 

 of accurate circular concentric etched lines, in which the 

 distances of the spaces between any two lines are in a high 

 degree equal. With a machine which does not allow of any 

 doubt as to the requisite accuracy, I have executed such an 

 etching of circular lines, that when it is placed before the object 

 glass of the telescope the light entering through a circular 

 aperture in the shutter, and falling perpendicularly upon it, 

 it shows concentric spectra in the telescope, which exhibits the 

 fixed lines.-f- Their distances from the axis are in the same 

 proportion as in the spectra, which are produced through the 



* See Note B in page 262. t See Note C in page 262. 



