250 Stellar Photography. [May, 
about ten hours, quickly causes a displacement of the details 
visible at the time of observation. 
With Saturn the image is so small that we cannot hope 
at present for much information from the photographs ob- 
tained of them. 
The curious objects scattered up and down the heavens, 
and called nebulae, form a much more likely class. The 
difficulty of drawing these objects is very great. Different 
observers are struck with different detail, and the keener- 
sighted observer will give an entirely different form to some 
wisp or patch which is figured by both. Hence the various 
drawings of the great nebula in Orion seem to show 
the light grasping powers of the instruments used, and the 
keenness of the observer, rather than an aCtual change in 
the nebula itself. If, then, the nebula could be photographed 
from time to time, there would be a means at once of deter- 
mining whether any change had taken place. This has 
actually been done by Dr. Draper in America, and by M. 
Common in England, both using large reflectors. M. Com- 
mon’s photograph, being taken with the largest aperture, 
shows of course the greatest amount of detail, but the glare 
from the brighter stars destroys the photograph of the parts 
of the nebula near them. 
But besides this field there is yet another where valuable 
work may be done. The photographic plate is most sensi- 
tive in the violet part of the speCtrum, and least sensitive in 
the red. Hence the image impressed upon the plate will be 
the result of the violet light emitted or reflected from that 
objeCt. Supposing two flames, one of red and the other of 
violet colour, were photographed on the same plate with the 
same exposure, although in intensity to the eye the red 
might be far the brightest, yet, since the plate is most sensi- 
tive to the violet, the brightness of the images on the plate 
would be reversed, the violet one coming out many times 
brighter than the red one. Now the stars are of various 
colours, some white, others blue, some red. If, then, the 
stars were photographed, the intensity of the image on the 
plate — just as in the case of the two flames — would be 
greatest for those stars which emitted most violet light. In 
this way it would be possible to obtain an accurate record of 
the violet light in the stars. 
Again, the violet light is chemical light. All life, animal 
and vegetable, depends upon this chemical light. If less 
than a certain amount, all life would cease in all probability ; 
while, on the other hand, if greater, its very intensity would 
kill plants and animals, or else produce a very exaggerated 
