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 XXII. Intelligence and Miscellaneous Articles. 



PHOTOGRAPHY AT THE LEAST-REFRANGIBLE END OF THE SOLAR 



SPECTRUM. BY ('APT. ABNEY, R.E., F.R.S. 



NEABLY two years ago I had the honour of reading a preliminary 

 note on photographing the least-refrangible end of the spec- 

 trum ; and it seems that the time has come when I ought to redeem 

 the promise implied by a " preliminary note " and enter further 

 into the subject. Last year, owing to a change in residence, I was 

 unable to pursue the subject with any degree of activity; but 

 during this last whiter and the present spring I have made fair 

 progress in my researches, the results of which I lay before the 

 meeting. These results are principally photographs themselves ; 

 and the first to which I shall call attention is one taken through 

 three prisms of dense flint glass, each of which had a vertical 

 angle of 62°. They Mere placed at the angle of minimum devia- 

 tion of B, and kept so. The focal lengths of the collimator and 

 camera were 18 inches and 2 feet respectively ; and a condensing 

 lens of 6 feet focus was employed to collect the light, the middle 

 of the collimating lens alone being filled with solar rays. In front 

 of the slit was placed a plate of orange glass, in order to cut off 

 the suffused bine rays, which experience had previously taught me 

 were inimical to the production of good negatives, owing to the 

 light dispersed in the prisms themselves. It will be noticed to 

 what an enormous distance below A the impressions of the bands 

 in the ultra red are to found. If the wave-lengths be used as 

 abscissa) and the measured distances of the known lines be used 

 as ordinates, it will be found, if the waves be completed by hand, 

 that a wave-length of not less than 10,400 tenth metres is im- 

 pressed, lloughly speaking, A is 7600 tenth, metres, and D 

 5900 tenth metres — by which it will be seen that I Mas within the 

 mark when I announced that I had obtained photographs as much 

 below A as D was above it. Xow this negative, though interesting 

 as a feat in photography, has no practical scientific value, as the 

 ultra red is so tremendously compressed that the absolute wave- 

 lengths could not be obtained from it. 



About the time I read my last paper, Captain Tupman kindly 

 lent me a speculum -metal grating, by Eutherford, having about 

 8600 lines to the inch ; but it was only lately, after removing its 

 glass covering, that I was fully able to appreciate its value. In 

 all gratings the red, or rather the ultra red, of the first order is 

 overlapped by the ultra violet and violet of the second order, and 

 the higher the order the more overlap there is. To remedy this, 

 which was a defect for the purpose for which I required it, I 

 placed before the slit of the collimator red glass, which completely 

 cut off the yellow and only allowed a little of the green to pass. 

 The prisms were replaced by the grating, and a photograph of the 



