170 DESCRIPTION OF THE FIXED LINES. 



the object-glass of a telescope, and brought to a focus at the distance of six feet six 

 inches, at which place an arrangement was adjusted for exposing white paper screens, 

 on which the more prominent fixed lines might be seen, and their position marked, or 

 sensitive plates substituted for the screens, occupying precisely the same position. The 

 lines on the screens could, therefore, be compared with those on the sensitive surfaces, 

 as to position and magnitude, with considerable accuracy. In these trials I have gen- 

 erally used an achromatic lens, but the lines can be beautifully seen by employing a 

 common double convex, if the screen be inclined forward in the way described in (663). 

 Either way answers very well. 



739. In order to identify these lines, I have made use of the map of the spectrum, 

 published by Professor POWELL, in the Report of the British Association for 1839. 

 With the instrumental arrangement described they are exceedingly distinct, and no dif- 

 ficulty arises in the identification of the more prominent ones. The spectrum with 

 which I have worked occupies upon the screen a space of nearly four inches and a 

 quarter in length from the red to the violet, or, more correctly speaking, from the ray 

 marked in that map A to the one marked k. In stating, however, that no difficulty 

 arises in identifying these lines, I ought to add that I am referring to that particular 

 map. In the figure annexed to Sir JOHN HERSCHEL'S Treatise on Light, in the Ency- 

 clopedia Metropolitan a, the ray marked G seems to differ from that of the report. But 

 Professor POWELL'S map being drawn from his personal observations, and with refer- 

 ence to these very difficulties, as it coincides with my own observations and measures, 

 I have employed it, and, therefore, take the letters he gives. 



740. It will be understood that the whole spectrum and all its lines cannot be obtained 

 at one impression. The difficulty which is in the way of effecting this rests in the 

 circumstance that different regions of the spectrum act with different power in pro- 

 ducing the proper effect. Thus, if on common yellow iodide of silver the attempt were 

 made to procure all the lines at one trial, it would be found that the blue region would 

 have passed to a state of high solarization, and all its fine lines become extinguished 

 by being overdone long before any well-marked action could be traced at the less re- 

 frangible extremity. We have, therefore, to examine the different regions in succes- 

 sion, exposing the sensitive surface to each for a suitable length of time. 



741. In fig. 103, I have given on the left side a representation of the larger lines of 

 FRAUNHOFER, the letters being derived, as has been said, from Professor POWELL'S map. 

 The position of the lines is, howerer, copied from my own spectrum as closely as I 

 have been able to accomplish it. 



742. In order that a comparison may be made between the new system of lines and 

 those of FRAUNHOFER, the right side of the plate gives a tithonographic representation 

 of them as obtained on a Daguerreotype plate which has been iodized to the yellow, 

 brought by the vapour of bromine to the red, and then slightly exposed to the vapour 

 of chloride of iodine. The map is so adjusted in the plate as to have its lines by the side 

 of those of FRAUNHOFFR which have the same name. Referring, therefore, to the plate, it 

 will be seen that there are, beyond the red ray, three extra-spectral lines, which I have 

 marked , A y. These, however, I have only occasionally found, for, from the general 



