16 
IOWA ACADEMY OF SCIENCES 
one cannot stand at the camera for much longer than fifteen minutes at 
a time, and must he relieved by a second observer. If one night is not 
sufficient for the exposure, the camera can be closed, the telescope turned 
into its usual position, and when the next clear night arrives, the tele- 
scope can be pointed to the same place, and the images superimposed 
■with perfect exactness. Thus the long hours of exposure may be con- 
tinued from night to night. 
Fig. 4 is a reproduction of the results of an eight-hour exposure on 
the beautiful nebula in Orion. While much of the nebula and fine details 
in the original negative are entirely lost, there is still more to be seen 
than could possibly be discerned by the eye in the same telescope. Many 
fine drawings of this nebula have been made by eminent astronomers; but 
the autographic record made on a sensitive plate is so vastly superior 
to anything that has been done by eye and hand that photography is said 
“to have definitely assumed the office of historiographer to the nebula”. 
The extent of the prodigious object had not been guessed at until the 
camera exposed its true form and outlying appendages. Portions which 
are now’ known to belong to the same mass w'ere catalogued as separate 
nebulae. 
The photographs of stars and star-clusters are even more wmnderful 
and interesting than those of the nebulae. Fig. 5 shows the result of a 
tw’o-hour exposure on the Double Star-cluster, in Perseus. The central 
portions are not so clearly resolved as they would be in direct vision; 
but the number and extent of the stars belonging to the cluster could 
never have been known by visual methods alone. By comparison with 
photographs taken in the past, or those to be taken in the future, any 
change in the cluster, either to-ward condensation or disintegration, could 
be readily noted. 
Pictures of the moon were among the earliest results of celestial 
photography. The old question of lunar change seems to have neen solved 
by the chemical eye. “Henceforth, at any rate,” as Miss Gierke so 
beautifully puts it, “'the lunar volcanoes can scarcely, without notice 
taken, breath hard in their age-long sleep.” In Fig. G„ the crescent is 
showui as taken by means of the three-inch amplifying lens. The lens is 
the work of Dr. John A. Brashear. Fig. 7, however, was originally 
taken without amplification, the image of the whole moon being less 
than one inch, in diameter. The negative was then enlarged to six inches 
in diameter. 
The field for research w’ork along the lines of celestial photography 
is unlimited; and the ambitious young astronomer finds here a new and 
promising guide for his energies. 
