ASTRONOMICAL PHOTOGRAPHY 759 



and the Sickle in the constellation of Leo for the November meteors. The battery of 

 cameras is easily attached to an axis which will be driven by clockwork so that the 

 stars in the region being photographed will appear as point images and afford a back- 

 ground for determining positions of the meteor trails as they may occur. The flasli 

 of a meteor across the field of view will leave its impression as a streak on the photo- 

 graphic plate. Frequently such a streak will be of nonuniform brightness, depending 

 upon sporadic changes in the brightness of the meteor itself. 



Photography is the one exact means for determining with accuracy the positions 

 of meteor trails from different points of observation. A comparison of plates simul- 

 taneously exposed at two stations a few miles apart makes possible the determination 

 of the altitude above the earth at which the meteor first became visible and also fre- 

 quently the altitude at which the meteor vanished. The mathematical calculation 

 of the altitude of meteor trails from such photographic observations is beyond the 

 scope of this book, and the reader is referred to the more extended treatises on this 

 branch of the subject. 



Photography of the Planets. — Photography of the planets, with the possible exception 

 of Jupiter and Saturn is rather beyond the scope of the amateur. In planetary 

 photography an auxiliary enlarging lens is employed in the optical axis of the telescope 

 objective and somewhat inside the focal plane. Because of the variation in "seeing" 

 conditions and the necessity for satisfactory moments when the atmosphere is particu- 

 larly tranquil, an auxiliary visual telescope is a prerequisite to planetary photography. 

 A shutter which may be quickly operated by a bulb or finger release is also essential. 

 The double-slide plateholder is a convenience in planetary photography for moving the 

 plate between exposures. Since at best a planetary image is exceedingly small, multi- 

 ple exposures may be made on a single plate, the images being duly separated by 

 appropriate movements of the double-slide plate carriage between exposures. The 

 observer, who is scrutinizing the planet through the visual telescope, awaits the 

 supreme moment which is well known to all seasoned obsei-vers. At such times 

 planetary detail may be caught at its best during the few brief seconds when atmos- 

 pheric disturbances are at a minimum. At this moment the shutter is released for 

 appropriate exposure but is instantly closed if the planetary image becomes blurred 

 through atmospheric turbulence. Extraordinarily good photographs of Jupiter, 

 Saturn, and Mars under favorable conditions have been taken at the Lick and Lowell 

 Observatories. The Lick Observatory located on Mount Hamilton, Calif., and the 

 Lowell Observatory at Flagstaff, Ariz., are particularly well favored climatically for 

 planetary photography. 



Various filters are frequently einployed to bring out planetary details. Photo- 

 graphs of Mars made in blue and red light on plates sensitive to these colors invariably 

 show wide differences in the characteristics. It appears that with blue-sensitive plates 

 light is reflected or scattered chiefly from the atmosphere surrounding the planet, 

 giving a somewhat diffuse image. In the case of photography with red light, the 

 filtered rays are those that have penetrated more extensively the atmosphere of the 

 planet and are reflected more completely from the surface of the planet, rendering 

 visible a large amount of surface detail that it is not possible to photograph in the use 

 of ordinary plates which are blue sensitive. For further details on planetary photog- 

 raphy, the reader is referred to publications on these subjects issued by the Lick 

 Observatory of Mount Hamilton, Calif., and the Lowell Observatory of Flagstaff, 

 Ariz. A good photographic image of any of the planets will stand enlargment from 

 the original negative several times. By repeated copying, contrast may also be 

 enhanced. 



Eclipse Photography. — One of the primary objects in expeditions to observe total 

 eclipses of the sun is the photography of the solar corona, that gaseous appendage to 



