50 ON THE CONSTRUCTION AND USE OF 



of exposure is too long, and second, there is a certain amount of lateral diffusion 

 in the thickness of the film, and in consequence a degree of sharpness inferior to 

 that of the image produced by the parabolic mirror. The shortest time in which 

 the moon has been taken in this observatory has been one-third of a second, on the 

 twenty-first day, but on that occasion the sky was singularly clear, and the intrinsic 

 splendor of the light great. The full moon under the same circumstances would 

 have required a much shorter exposure. A person, however, who has put his eye 

 at the focus of such a silvered mirror will not be surprised at the shortness of the 

 time needed for impressing the bromo-iodide film ; the brilliancy is so great that it 

 impairs vision, and for a long time the exposed eye fails to distinguish any moder- 

 ately illuminated object. The light from 188 square inches of an almost total 

 reflecthig surface is condensed upon 2 square inches of sensitive plate. 



Occasionally a condition of the sky, the reverse of that mentioned above, occurs. 

 The moon assumes a pale yellow color, and will continue to be of that non-actinic 

 tint for a month or six weeks. This phenomenon is not confined to special localities, 

 but may extend over great tracts of country. In August, 1862, when our regiment 

 was encamped in Virginia, at Harper's Ferry, the atmosphere was in this condition 

 there, and was also similarly aftected at the observatory, more than 200 miles dis- 

 tant. As to the cause, it was not forest or prairie fires, for none of them of sufii- 

 cient magnitude and duration occurred, but was probably dust in a state of minute 

 division. No continued rain fell for several weeks, and the clay of the Virginia roads 

 was turned into a fine powder for a depth of many inches. The Upper Potomac 

 river was so low that it could be crossed dry-shod. On a subsequent occasion when 

 the same state of things occurred again, I exposed a series of plates (whose sensi- 

 tiveness was not less than usual, as was proved by a standard artificial flame) to the 

 image of the full moon in the 15g inch reflector for 20 seconds, and yet obtained 

 only a moderately intense picture. This was 40 times as long as common. 



Upon all photographic pictures of celestial objects the influence of the atmosphere 

 is seen, being sometimes greater and sometimes less. To obtain the best impres- 

 sions, just as steady a night is necessary as for critical observations. If the image 

 of Jupiter is allowed to pass across a sensitive plate, a streak almost as wide as the 

 planet is left. It is easily seen not to be continuous, as it would have been were 

 there no atmospheric disturbances, but composed of a set of partially isolated images. 

 Besides this planet, I have also taken impressions of Venus, Mars, double stars, &c. 



An attempt has been made to overcome lateral diff'usion in the thickness of the 

 film by the use of dry coUodion plates, more particularly those of Major Russel and 

 Dr. Hill Norris. These present, it is true, a fine and very thin film during exposure, 

 but while developing are so changed by wetting in their mechanical condition that 

 no advantage has resiilted. It was while trying them, that I ascertained the great 

 control that hot water exercises over the rapidity of development, and time of expo- 

 sure, owing partly no doubt to increase of permeability in the collodion film, but 

 also partly to the fact that chemical decompositions go on more rapidly at higher 

 temperatures. I have attempted in vain to develop a tannin plate Avhen it and the 

 solutions used were at 32° F., and this though it had had a hundred times the exposure 

 to light that was demanded when the plate was kept at 140° F. by warm water. 



