Celestial Photography. 133 



phenomena. A rigid observation and description of fact is 

 one of the most difficult arts for a man to acquire, and few are 

 the minds that keep a due balance of their powers when an 

 epidemic excitement tends to give undue force to wonder and 

 imagination. 



CELESTIAL PHOTOGRAPHY.— ENGELMANN ON 

 DOUBLE STARS.— CRIMSON STAR. 



BY THE EEV. T. W. WEBB, A.M., E.E.A.S. 



In a recent number of the Intellectual Obseevee an account 

 was given of Dr. Draper's Great Reflector with a silvered 

 glass speculum 15^ inches in diameter, intended chiefly for the 

 purpose of celestial photography. We now propose to give 

 some details as to the mode of its application. 



The first photographic picture of the Moon appears to have 

 been taken by his father in the year 1840, when a Daguerreotype 

 image was obtained by a 20-minutes' exposure, Daguerre 

 himself having failed in a similar attempt. This experiment 

 was made by means of a heliostat, or apparatus for reflecting 

 the incident rays always in one direction, notwithstanding the 

 motion of the body whence they flow. Dr. Draper, Jan., has 

 preferred an arrangement in which, the telescope remaining 

 fixed, and the lunar image being allowed to pursue its own 

 course, the collodion plate is caused to follow its path accurately 

 by means of an appropriate moving power. Mr. De la Rue 

 first suggested this ingenious mode of counteracting the earth's 

 diurnal motion, and it has been carried into effect by the Earl 

 of Rosse ; but the mechanism employed by Dr. Draper was 

 devised independently by himself and his brother. An attempt 

 to move the slide carrying the collodion plate by means of 

 screws turned by hand failed from the communicated tremors. 

 A " sand-clock ;; which was substituted as prime mover, and 

 which caused the frame to slide along two rods, answered 

 much better, but an ingenious contrivance by which rolling 

 was substituted for sliding friction was a still greater improve- 

 ment, and admitted of the image being followed steadily for 

 nearly four minutes, though fifty seconds were all that was ever 

 found necessary. Two cross lines traced on ground glass are 

 used as a preliminary guide in order to fix the direction of the 

 apparatus parallel to that of the moon's motion, and when a 

 lunar crater stands steadily on their intersection for twice or 

 thrice the period required for exposure, the adjustment is 



