DIFFICULTY OF OBTAINING THEM IN THE YELLOW AND GREEN. 



diminution of effect in that region, they do not always come out in a plain and striking 

 manner. None of FRAUNHOFER'S lines in the yellow and green are given, but G and its 

 companions are very strongly marked, as also the group about i. But by far the most 

 striking in the whole tithonograph are those marked H and k ; and now, passing beyond 

 the violet, and out of the visible limits of the spectrum, four very striking groups make 

 their appearance. The first line of each of these groups I have marked, in continuation 

 of FRAUNHOFER'S nomenclature, M, N, O, P. In L there are three lines, in M five, in 

 N three, in O three, and in P five. 



743. Besides these larger groups, the whole tithonograph is crossed by hundreds of 

 minuter ones, so that it is utterly impossible to count them. If, as it has been said, 

 nearly 600 have been counted between A and H, I should think there must be quite 

 as many between H and P. In speaking, therefore, of these lines as though they were 

 strong individual ones, the expression is to be taken with some limitation. It is quite 

 likely that each of those bolder lines is made up of a great number that are excessively 

 narrow and close together. 



744. If the absorptive action of the sun's atmosphere be the cause of this phenome- 

 non, that action takes place much more powerfully on the more refrangible and extra- 

 spectral region. The lines exhibited there are bold and strongly developed ; they are 

 crowded in groups together. 



745. I cannot doubt, judging from analogy, that, by proper modes of investigation, 

 similar lines might be detected in the calorific extra-spectral region. 



746. The contrast between the visible and tithonographic spectra is maintained by 

 the non-appearance of lines in the yellow and green regions. Once only I thought I 

 perceived a line corresponding to FRAUNHOFER'S F, but it was exceedingly faint, and, 

 on the whole, doubtful. 



747. FRAUNHOFER'S lines, which occur on the orange, yellow, and green spaces, thus 

 leaving no corresponding impression, another argument is furnished of the independence 

 of the tithonic and luminous rays. It is probable that more perfect arrangements than 

 I have used would give the ivhole spectrum as though it were full of these inactive 

 spaces ; and in stating that nothing like FRAUNHOFER'S lines exist in those medial re- 

 gions, I therefore simply wish it to be understood that I can find nothing at all cor- 

 responding in magnitude to the great lines marked D, E, F, though hundreds of micro- 

 scopic ones may probably exist in these very spaces. 



748. The position of the lines as represented on the sensitive surface is found to be, 

 as might have been anticipated, independent of the chemical nature of that surface. 

 The iodide of silver gives them in the same places as the bromide. 



749. An argument might be drawn, as has been said, from the absence of these lines 

 in the yellow and green spaces, as to the independence of the dark rays and light. 

 This is, however, only another proof of a fat of which we have now abundant evidence. 

 In 1834, when my attention was first fixed forcibly on these things, and I began to 

 make prismatic analyses by the aid of sensitive paper, some of my first trials were di- 

 rected to the detection of these fixed lines. At that time I was employing sensitive 

 paper made with the bromide of silver, precisely as has been subsequently done in Eu- 



