MODE OF PRODUCING THE FIXED LINES. 



CHAPTER XIV. 



ON A NEW SYSTEM OF INACTIVE TITHONOGRAPHIC SPACES IN THE SOLAR SPECTRUM 

 ANALOGOUS TO THE FIXED LINES OF FRAUNHOFER, AND ON THE TITHONOTYPE. 



(From the London, Edinburgh, and Dublin Philosophical Magazine for May, 1843.) 



CONTENTS : Mode of producing the fixed Lines. -Description of them. Difficulty of 



obtaining them in the Yellow and Green. 



Daguerreotypes are dotted Surfaces. Mode of copying them by the Tithonotype. Po- 

 L larized Structure of the Daguerreotype Film, 



734. WHEN a beam of the sun's light, directed horizontally by a heliostat, is thrown 

 into a dark room, and, passing through a chink with parallel sides, is received on the 

 surface of a homogeneous flint-glass prism, which refracts it at the angle of minimum 

 deviation, and, after its passage through the prism, is converged to a focal image on a 

 white screen by the action of an achromatic lens, the spectrum which results is given 

 in great purity, and FRAUNHOFER'S lines are quite apparent. The larger ones are seen 

 by the most casual inspection. 



735. A tithonographic surface, after being placed in this spectrum, exhibits impres- 

 sions of an analogous character, being covered with the representations of multitudes 

 of inactive lines varying greatly in dimensions. 



736. After several attempts last summer, I succeeded in discovering these lines, and 

 have obtained impressions of them sufficiently perfect. 



737. Before proceeding to the description of the mode which is to be followed, and 

 of the characters of the lines themselves, I cannot avoid calling attention to the re- 

 markable circumstance, which has frequently presented itself to me, of a great change in 

 the relative visibility of FRAUNHOFER'S lines, when seen at different periods. There are 

 times at which the strong lines seen in a red ray are so feeble that the eye can barely 

 catch them, and then, again, they come out as dark as though marked in India ink on the 

 paper. During these changes, the other lines may or may not undergo corresponding 

 variations. The same observation equally applies to the blue and yellow rays. It has 

 seemed to me that the lines in the red are more visible as the sun approaches the hori- 

 zon, and those at the more refrangible end of the spectrum are obvious in the middle of 

 the day. 



738. A beam of the sun, passing horizontally from a heliostat mirror into a dark 

 room, was received on a screen with a slit in its centre, the slit being formed by a pair 

 of parallel knife edges, one of which was movable by a micrometer screw ; the instru- 

 ment being, in fact, the common instrument used for showing diffracted fringes. The 

 screw was adjusted so as to give an aperture j? inch wide, and the light, passing through, 

 fell upon an equiangular flint-glass prism placed at the distance of eleven feet Imme- 

 diately on the posterior face of the prism,, the ray was received on an achromatic lens, 



Y 



