lOi M. Fraunhofer on tJic Laws 



and their light, like that of these rings, is not homogeneous. 

 In my Treatise I have denominated these spectra, which are 

 seen through a single aperture, spectra of an external hind, in 

 order to distinguish them from those which are produced in 

 another manner. In future, however, I shall denote them by 

 the appellation of spectra of the first class^ in order to speak 

 on this subject with greater facility. If, on the contrary, light 

 is inflected through a great number of small apertures all at 

 equal distances, then from the reciprocal action of the inflected 

 rays on each other, when observed with a telescope, spectra of 

 another sort are produced, of which the light is perfectly ho- 

 mogeneous, provided there is a sufficient number of the small 

 apertures. And in these are perceived the same fixed lines 

 and streaks which are seen in a spectrum produced by a prism 

 when observed by a telescope. By this means the latter kind 

 of spectra are peculiarly fitted for discovering the law of these 

 modifications of light ; for by observations made on these lines 

 and stripes that law may be deduced with a high degree of 

 accuracy. Such a system of equal small parallel intervals can 

 be most easily procured, either by pressing thin silver or gold 

 wire into the threads of a very fine screw, or by etching paral- 

 lel lines upon a plate of glass covered with gold leaf. In these 

 experiments the light of the sun must enter a darkened room 

 by a vertical aperture of extremely small apparent width in 

 the shutter, and fall upon the object-glass of a telescope, in 

 the direction of its axis,placed at the opposite end of the room. 

 When the observer has drawn out the eye-glass of the tele- 

 scope so far that he sees the aperture in the shutter as dis- 

 tinctly as possible, and then places the parallel wires in such 

 a manner before the object-glass that the threads shall run 

 horizontally, he sees the aperture itself unaltered, as if there 

 were no wires ; but at the same time there will appear to him, 

 at some distance from this aperture in the shutter, and in a 

 horizontal direction, symmetrically on both sides, very in- 

 tensely coloured spectra, which are repeated, and become 

 wider in proportion to their distance from the middle (or axis,) 

 so that the first repetition has twice the breadth, the second 

 three times, and so forth ; and in the same proportion as their 

 distances from the axis are increased. These spectra I deno- 



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