404 H. Draper—Diffraction Spectrum Photography. 
different types of force in three different but overlapping regions. 
Heat was supposed to be principally found at the less refrangi- 
ble end, light in the middle, ae actinism at the more refrangi- 
ble. But he showed that this error has partly arisen from using 
prismatic spectra, which condense the red end and dilate the 
violet, and do not present the rays in the true order of their 
wave-lengths, and partly from the nature of our ordinary photo- 
graphic substances. He proved that actinism, or the power of 
chemical decomposition, does not particularly belong to the vio- 
let end of the spectrum,. but is found throughout its whole 
length. But bromide and iodide of silver, as used in collodion 
photography, are more readily 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 actinism, but it is a series of ethereal 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 excess 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 
suitable times during the exposure. The region from wave 
length 4000 to 4850 only requires about one-tenth of the ume 
demanded by that from 3440 to 8510. In the negative which 
produced the accompanying plate, the line O had 16 minutes 
and G 24 minutes, and the former is under-exposed. These 
exposures seem at first sight unusually long for a wet collodion 
surface, but it must be remembered that the slit used was only 
tts Of an inch wide, and that the diffraction grating gives , h 
almost complete circle of spectra round itself, amongst et 
this thin band of light is divided. A beam ;}z of an me 
(00028 meter) wide is spread out in this case into strea 
about 78 feet (23°77 meters) long. 
was undoubtedly correct, the paper proof of it which I had, oe 
stretched unequally in printing, and on applying a photogr? o 
reduction to my spectra, coincidence could not be obtal 
