126 LINES IN THE SPECTEUM. 



very narrow in tlie direction of the plane of refraction, but broad parallel to tbe 

 axis of the prism, five well-defined dark straight lines could be distinguished 

 crossing the spectrum at right angles, and maintaining invariably the same posi- 

 tions relatively to the colors. This number he afterwards increased to seven. 

 These lines may very easily be distinguished by holding a prism near the eye, 

 parallel to any small fissure through which light makes its way into a dark 

 room. The reason they escaped the notice of Newton and other earlier ob- 

 servers is to be found in the fact that those observers employed a pencil so 

 broad iu the direction of refraction as to make the actually observed spectrum a 

 compound of many superposed and unconformable spectra, thereby obliterating 

 these very narrow markings. In fact, every point in an aperture of sensible 

 magnitude, through which the light experimented on is introduced into the dark 

 room, produces a spectrum of its own. Moreover, supposing that it is the sun- 

 light which is introduced through the aperture ; it may be said that every point 

 of the aperture produces not only one spectrum, but as many spectra as there 

 ai'e points in the sun's disk from which lines may be drawn to the assumed 

 point in the aperture. As all these lines, so drawn, would, in the absence of the 

 prism, produce a white circular image of the sun upon the screen in the dark 

 room, having a diameter increasing with the distance of the screen from the 

 aperture, it follows that, when the prism is introduced, the spectrum produced 

 by each point of the aperture will have a breadth equal to the diameter of this 

 white image of the sun, and that its elongated form is due to the lateral unequal 

 displacement of an indefinite number of circles, produced by the several elemen- 

 tary rays of which white light is made up. The interposition of a convex lens 

 between the prism and the aperture may serve to reduce the breadth and sharpen 

 the boundary of the image ; but still it is manifest that with a circular aperture, 

 there must, unless the diameter is made too small for convenient observation, be 

 a considerable mixture of rays of different refrangibility in every part of the 

 length. It is therefore best, for the purpose of obtaining a spectrum at once 

 broad and pure, to employ an aperture very narrow in the plane of refe-action, 

 and broad in the direction of the axis of the prism. This may be still further 

 improved by the use of a convex lens of long focus, as above described ; or bet- 

 ter, by the use of a cylindrical lens, with its cylindrical axis parallel to the length 

 of the aperture. With an arrangement like this, the lines of Dr. "WoUaston may 

 be easily exhibited, and many more. By aiding the eye with a telescope, the 

 number discovered becomes surprisingly great. Mr. Fraunhofer, of Munich, 

 enumerated five hundred and ninety, and Sir David Brewster afterward in- 

 ci-eased this number to two thousand. Their general appearance under the tele- 

 scope is shown in the figure annexed. 



Wy \ T\ 





[i_il li5!]ii,- iiL !.feuii.i iiLLiiuu:; JuJIll iuillLfetiii'L ij-ifa. - jaai 



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BED, ORANGE YELLOW GREEN BLUE INDIGO VIOLET 



Fig. 13. 



The eight principal lines are distinguished by the letters A to II, of which 

 the line A is at the beginning of the red, and the line II about the middle of the 

 violet. The line A does not appear in the figure. The positions of these lines 

 being definitely fixed among the colors of the spectrum, they furnish valuable 

 aid in comparing the refracting powers of different bodies, and have served to 

 reveal the fact that bodies whose mean refractive powers are equal, do not always 

 equally refract the several elementary rays. The line A is not among the most 

 easily discernible, but Sir David Brewster has discovered others in the almost 

 imperceptible light below A ; and Sir John Ilerschel, and especially Professor 



