1882.] 



On the Reversal of the Metallic Twines, fyc. 



85 



out the most characteristic lines without the additional diffused rays of 

 the air- spectrum ; at the same time very delicate and feeble air-lines 

 are adequately shown. This has always been accomplished by making 

 a series of comparative exposures. With gelatine emulsion dry plates 

 great latitude in exposure is capable of yielding perfectly satisfactory 

 photographs. An under-exposed plate is not easy to develop, in order 

 that the usual density for the strong lines as seen in a good negative 

 may be gained. The air-lines are generally very feeble or altogether 

 omitted. An over-exposed plate is likewise difficult to develop ; it 

 yields a thin flat image, and more or less marked indications of a con- 

 tinuous air-spectrum are seen. 



Over-exposure, even when not excessive, is liable to cause strong 

 lines to appear reversed. I have mentioned in my paper " Notes on 

 Certain Photographs of the Ultra- Violet Spectra of Elementary 

 Bodies" ("Journal of the Chemical Society," vol. xli, p. 89, 1882), 

 that sometimes lines appear reversed in one photograph, but not in 

 another. This did not seem at all likely, or even possible, to be caused 

 by over-exposure, because the two periods differed only by a minute ; 

 but I have small doubts now on the matter. The conversion of what is 

 called a negative into a positive image by excessive exposure has been 

 already noticed by Mr. C. Bennett (" British Journal of Photography," 

 1878), by Captain Abney, who investigated the nature of the change 

 ("Phil. Mag." [5] 10, p. 200), occurring in the sensitive film, and by 

 M. Janssen (" Comptes Rendus," 90, pp. 1447 — 1448). 



In illustration of this phenomenon, I may mention a remarkable 

 result I obtained on one occasion when photographing a landscape. I 

 endeavoured to secure a picture with detail in a shaded foreground, 

 and a direct view of the setting sun, with mountains in the middle 

 distance, and strongly illuminated as well as dark clouds. In one case 

 I succeeded remarkably well, but in another plate the foreground was 

 good, but the sun was completely reversed. The negative image was 

 clear glass and the sun printed black. What should have been a 

 negative in the strong lights became a positive. Again, by exposing a 

 plate to the cadmium spectrum, the whole of the metallic lines were 

 rendered distinctly, but with a flatness and want of density, the whole 

 of the strong air- lines at the least refrangible end of the spectrum 

 were, however, completely reversed. 



Any strong lines may be reversed by over-exposure without materially 

 altering the appearance of the rest of the spectrum. This is particularly 

 the case with the lines of the metals magnesium, aluminium, and 

 indium, but particularly so with magnesium. The reversal takes 

 place in the centre of the line, that is to say, where the radiation is 

 most active. Except by the method of comparative exposures, which 

 I have always employed, it would be impossible to say whether a 

 reversal was due to an absorbed ray or an over-exposed plate. 



