362 Prof. Draper on inactive Tithonographic Spaces in the 
beautifully seen by employing a common double convex if the 
screen he inclined forward in the way described in this Journal 
for December 1842, Either way answers very well. 
6. In order to identify these lines I have made use of the 
map of the spectrum published by Prof. Powell in the Report 
of the British Association for 1839. With the instrumental 
arrangement described they are exceedingly distinct, and no 
difficulty arises in the identification of the more prominent 
ones. The spectrum, with which I have worked, occupies upon 
the sereen a space of nearly four inches and a quarter in length 
frora the red to the violet, or, more correctly speaking, from 
the ray marked in that map A to the one marked &, 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 J. Herschel’s treatise on 
Light, in the Encyclopedia Metropolitana, the ray marked G 
seems to differ from that of the Report. But Prof. Powell’s 
map being drawn from his personal observations, and with 
reference to these very difficulties, as it coincides with my own 
observations and measures, I have employed it and therefore 
take the letters he gives. 
7. It wiil 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 producing 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 be- 
come extinguished by being overdone long before any well- 
marked action could be traced at the less refrangible. ex- 
tremity. We have therefore to examine the different regions 
in succession, exposing the sensitive surface to each for a suit~ 
able length of time. 
8. In the Plate which accompanies this paper [ Plate III. ], 
I have given on the left side a representation of the larger 
lines of Fraunhofer, the letters being derived, as has been 
said, from Prof. Powell’s map, The position of the lines is, 
howeyer, copied from my own spectrum as closely as I haye 
been able to accomplish it, 
9. 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 ob- 
tained 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 ehloride ofiodine. The 
