SOO l*rof. E. Wartmann oti some Nc'o Liiics 



by Cauchoix, magnifying seven times, with a focal distance of 

 C^'Ge, and an objective the diameter of which measures 

 C'l-OeO. The prism, the refracting angle of which is 45° 4' 20", 

 is arranged in the position of minimum deviation. 



" I thus discovered in the luminous field of the spectrum 

 projected horizontally, a great number of /o?2^2V?/(//?/rtZ straight 

 lines, some thicker than others, pai'allel to each other and to 

 the length of the spectrum. They are therefore perpendicular 

 to the transverse rays aheady known. They are perceptible 

 before the telescope is drawn out sufficiently for the latter to 

 be seen distinctly. I rarely succeeded in obtaining a view of 

 the two systems at once. These longitudinal lines do not 

 arise from an impurity in the prism or in the glasses of the 

 telescope, for they appear the same with other refractive ap- 

 paratus. Neither can they be attributed to inequalities in the 

 rectilinear margins of the opening, nor to an imperfect state 

 of the surface of the mirror. But their general aspect depends 

 upon the size of the aperture, on the distance and the position 

 of the prism, on the state of the atmosphere, on the height of 

 the sun above the horizon, and on the hour of observation. 

 It often appeared to me that several of them became displaced 

 and extinguished, as if the transparency of the air had sud- 

 denly changed. By varying the distance of the eye-glass from 

 the objective, they are partially, if not all, changed from black 

 into luminous lines, and vice versa. These appearances, not 

 having the fixity and the distinctive characters of the lines 

 figured by Fraunhofer, must be assigned to a different cause. 



" The longitudinal stripes, examined by the help of a very 

 excellent equilateral prism by Soleil, generally appear so much 

 the more visible as the aperture is larger. Whether it be of 

 a circular or rectangular form is of no consequence. By re- 

 Tioving the exterior mirror and employing direct light, the 

 lines are seen to varv in number and position, according as 

 the naked eye, placed immediately against the prism, moves 

 parallel to its vertical edges*. These appearances are owing 

 to tlie interference of the central rays with those which are 

 reflected against the edges of the opening. They disappear 

 when all reflexion is rendered impossible in this as in the 

 prism, and when the axis of the eye or of the glass is suitably 

 directed. They do not change when the light of a lamp is 



* The prism is ofsiich pure flint, that with direct hght, in misty weather, 

 it is possible to sec with the naked eye at least sixty of Fiauiihofer's lines. 

 Tiie two marked 13 and C in the red are perceived very distinctly; but the 

 bundle H, and especially that which follows it in the violet, are not clear. 

 The exijcrinient constantlv succeeded at six metres, with an aperture of 

 l"""-4. 



