RECORDS 95 



Summary of Papers. 



Photography was discovered in 1837 and the first astronomi- 

 cal photograph was taken in 1840 b}' Dr. Draper of New York. 

 It was a photograph of the moon made on a daguerreotype 

 plate and gave great promise for the future. Bond in 1850 

 made the first photographs of the stars. Rutherford of New 

 York in 1858 made some remarkable photographs of the moon, 

 and later some star photographs. 



Photography has now become so valuable in astronomy that 

 it is applied in every department. It is not true, howe\er, that 

 it will displace the eye. There are certain fields where the eye 

 will be superior to the photographic plate, but in many other 

 fields photography has led to results that never could have been 

 obtained by visual observation. I shall speak to-night of work 

 done at the Yerkes observatory with a telescope designed for 

 visual observation. It is fortunate that this telescope was not 

 designed for photography alone, for b\' the use of methods re- 

 cently devised it has been possible to use it for photography and 

 the results are not at all inferior to what they might have been 

 in a telescope designed for photography alone. 



The forty-inch telescope of the Yerkes Observatory can be 

 considered as a long camera with a focal length of about sixty- 

 four feet. Its field of view embraces a circle in the sky of only 

 about five minutes of arc in diameter. In photographing groups 

 and clusters of stars this long focal length makes it possible to 

 separate stars which would have been run together into one 

 mass with an instrument of shorter focal length. A means of 

 counteracting the uncorrected chromatic aberration has been 

 devised by Mr. Ritchie of the Yerkes Observatoiy. He em- 

 ploys a yellow collodion film in front of the photographic plate 

 at the eye end of the instrument, by which the blue rays are cut 

 off Suitable isochromatic plates such as can be found in the 

 market are used. This is a very inexpensive means of using the 

 telescope for photography. A special form of guiding apparatus 

 to keep the star image at the same point of the plate has to be 

 employed. On account of the unavoidable flexure of the large 

 telescope tube, an auxiliary telescope placed parallel to the 



