a Silvered-Glass Reflecting Telescope. 253 



vature) will fail to indicate any change in figure. The glass 

 covered by metal is even more durable than a lens. 



However, unless the astronomer can have access to a much 

 steadier atmosphere than prevails at Hastings near New York, 

 where my observatory is, there would be but slight inducement 

 to build a very large instrument. In the nine months' interval 

 between March and December 1863, only three really fine nights 

 occurred, when the best lunar negatives could be taken. Up to 

 the present date, September 1864, there has not been a single 

 occasion this year on which the results of the past one could be 

 exceeded. The good nights have been during the absence of the 

 moon. If the instrument could be transported to the Peruvian 

 plateaus, 15,000 feet above the sea, or somewhere near the 

 equator on the rainless west coast of South America, it could 

 accomplish more. 



Notwithstanding these impediments, I have succeeded, as has 

 already been stated, in making some photographs of the moon 

 50 inches in diameter, and many of smaller sizes. In order to 

 take advantage of a steady night when it does occur, it is neces- 

 sary to get photographs whenever it is clear, so as to keep the 

 chemicals and clock in the best order. For this reason about 

 1500 original negatives were made in 1862 and 1863. A sin- 

 gular cause has this summer led to the loss of a great deal of 

 time. Owing to an excessive drought, the woods have been on 

 fire in many places, and have communicated to the atmosphere 

 the power of stopping the larger portion of the chemical rays. 

 The moon of June 19 required an exposure of ten minutes to 

 secure an impression, which was not more vigorous than one of 

 that phase taken in ^^o tn °f the time usually is. On the next 

 night two seconds were sufficient. In the meantime there had 

 been a heavy dew, but no rain. These smoky atmospheres give 

 great prominence to the lines in the less refrangible parts of the 

 spectrum about A. The diminution of the light on another 

 occasion was so great that the eye could look without inconve- 

 nience upon the meridian sun, which, as it declined to the west, 

 was gradually extinguished, and this though there were no 

 clouds. These phenomena are by no means confined to small 

 areas, but extend over large tracts of country. When my regi- 

 ment was stationed at Harper's Ferry in Virginia in 1862, a 

 similar condition of atmosphere prevailed there and at the obser- 

 vatory 200 miles distant. The yellowness was at that time attri- 

 buted to dust in the air. 



In developing and enlarging photographs, care is necessary to 

 preserve the proper relation of light and shade. In the case of 

 the full moon, for instance, there is a tendency to flatness and 

 an indistinct appearance. If, however, the negative, instead of 



