420 Dr. H. Draper on Diffraction- Spectrum Photography. 



tographic part of the operation is the device for avoiding the un- 

 equal action on the sensitive plate of different rays of the spec- 

 trum. It has been commonly supposed until the recent memoirs 

 of J. W. Draper, that there are in the spectrum three different 

 types of force in three different but overlapping regions. Heat 

 was supposed to be principally found at the less refrangible end, 

 light in the middle, and actinism at the more refrangible. But 

 he showed that this error has partly arisen from using pris- 

 matic spectra, which condense the red end and dilate the violer, 

 and do not present the rays in the true order of their wave- 

 lengths, and partly from the nature of our ordinary photographic 

 substances. He proved that actinism, or the power of chemical 

 decomposition, does not particularly belong to the violet end of 

 the spectrum, but is found throughout its whole length. But 

 bromide and iodide of silver as used in collodion photography 

 are more easily decomposed by vibrations of certain lengths and 

 periods than by others ; and hence the excess of action seen at 

 the violet end is a function of certain silver compounds, and not 

 of the spectrum. Other substances, as carbonic acid, show 

 maxima elsewhere, as in the yellow region. The solar beam is 

 therefore not compounded of three forces, light, heat, and acti- 

 nism, but it is a series of sethereal vibrations which give rise to 

 one or other of these manifestations of force, depending on the 

 surface upon which it falls. 



In order to provide against this excees of action in certain 

 parts of the spectrum, I introduced a system of diaphragms 

 placed in the vicinity of the sensitive plate and removed at suit- 

 able times during the exposure. The region from wave-length 

 4000 to 4350 only requires about one tenth of the time de- 

 manded by that from 3440 to 3510. In the negative which 

 produced the accompanying Plate the line had 15 minutes, 

 and G 2^ minutes ; and the former is underexposed. These 

 exposures seem at first sight unusually long for a wet collodion 

 surface ; but it must be remembered that the slit used was only 

 y{ ( y of an inch wide, and that the diffraction-grating gives an 

 almost complete circle of spectra round itself, amongst which 

 this thin band of light is divided. A beam -fTo °^ an * n °k 

 ('00023 metre) wide is spread out in this case into a streak 

 about 78 feet (23*77 metres) long. 



After the production of spectra that were in focus from end 

 to end, it was next necessary to attach a scale to them by which 

 the wave-lengths might be read. At first I tried by reducing 



Angstrom's maps to the proper dimensions to accomplish this 

 object; but the undertaking proved to be difficult, and was un- 

 successful, because, though the original drawing on the stone 

 was undoubtedly correct, the paper proof of it, which I had, had 



