i882. 145 Trans. N. V. Ac. Scz. 



seeing. The glass was completed iu December, 1864; but it was not 

 until March 6 of the following year that a sufficiently clear atmo- 

 sphere occurred, and on that night the negatives were obtained, from 

 which the pictures of the moon that we shall see to-night were taken. 



But even with this method Mr. Rutherford was not satisfied. 

 Coming back to the iiX-inch object glass which he had used at first, 

 he determined to see whether or not the addition of a meniscus lens, 

 outside the front lens, would not give him the requisite shortness of 

 focus and bring the actinic rays absolutely together. By this arrange- 

 m.ent he got a telescope which can be used for all purposes of astro- 

 nomical research, and he has also eclipsed all his former efforts. — {Lock- 

 yer, Star-gazer, p. 467.) 



In the series of moon-pictures by Rutherford and Draper, we 

 have a complete map of the moon , not only are the grander outlines 

 clearly shown, but many of the smaller details. [Illustrations were given 

 of the work of Beer and Maedler, of Germany, and Schmidt, of 

 Athens. Comparisons were made between the photographs and eye- 

 drawings, and the use of photography in detecting changes in the 

 moon was alluded to.] 



In taking sun-photographs, it becomes necessary, on account of the 

 dazzling brightness of the sun's light, to make the exposure practically 

 instantaneous. Some astronomers reduce the aperture of their tele- 

 scopes, by suitable diaphragms, to about two inches diameter, thus 

 cutting off a great deal ot the sun's light. Even then, the time of ex- 

 posure is less than the three hundredth part of a second. [The method 

 of making these quick exposures by what is called the instantaneous slide 

 was explained. A series of pictures of the sun, by Mr. Rutherford, 

 was then thrown on the screen. The use of photography, in studying 

 the mottled surface of the sun, was illustrated by photographs from 

 Rutherford and Janssen, and drawmgs trom Nesmith, Secchi, 

 Muggins and Langley.] 



There are times when the sun shows us surroundings which it is im- 

 possible to see ordinarily, as at total solar eclipses. Such a time is an 

 exciting moment, the eclipse lasting from a few seconds to some six or 

 eight minutes only, and the average time being such that altogether we 

 have only a few days in a century for observation. Drawings made 

 by two observers, side by side, differ very peculiarly. Photography as- 

 sists us immensely in our study, and some of the best observations of 

 eclipses have thus been made. [Illustrations of the photographs ot 

 1851, i860, 1869, 1870 and 1872 were then thrown on the screen and 

 explained. A full description was given of the apparatus used by 

 the American parties, in photographing the transit of Venus, and some 

 illustrations of actual photographs taken were shown.] 



