6 4 The Smithsonian Institution 



in spectroscopy, the definition is the secondary and the 

 amount of light collected the primary consideration, large re- 

 flectors will have the advantage. If the difficulties due to 

 temperature, flexure, etc., are overcome, the reflector has 

 another advantage in that the rays of all wave-lengths are 

 brought to a single focus, which is not the case for the re- 

 fractor. This constitutes a marked advance in certain fields 

 of celestial photography. 



In 1857 Doctor Draper visited Lord Rosse's observatory at 

 Parsonstown, and was privileged to see the operations of 

 grinding and polishing mirrors of speculum metal. In 1858 

 he began the construction of a 15 -inch speculum in America. 

 In 1860 speculum-metal was abandoned for silver-on-glass. 

 During 1 86 1 three mirrors were constructed of 15^ inches 

 aperture, and others smaller. In 1862 Doctor Draper was 

 with his regiment in the Virginia campaign ; but in the winter 

 of that year two 15^-inch and two 9-inch mirrors were 

 completed. The greater part of the year 1863 was devoted 

 to lunar and planetary photography and to the enlargement 

 of focal negatives. 



The various practical processes required for the manufac- 

 ture of a perfect mirror are described in detail and at length. 

 These descriptions are of the highest value, for they embody 

 the results of long practical experience described by an ob- 

 server of adequate theoretical knowledge. The first of these 

 sections relates to experiments on a metal speculum. The 

 next section refers to silvering the glass, and contains many 

 practical hints. It is interesting to note that the film of sil- 

 ver is about 2 o (H) o o" f an mcn m thickness, and of the same 

 optical figure, therefore, as the glass surface. Its thinness 

 can be expressed in a striking form by remarking that the 

 value of the silver on a 15^ -inch mirror is not above one 

 cent. The durability of these films (when kept free from 



