224 STELLAR PHOTOGRAPHY. 



same way. It represents the Nebula of Orion, with exposures of 16™, 8"*, 4"*, 2"", 1'", 

 30'', and 15'. Besides measuring the relative brightness of the stars, this plate per- 

 mits the relative intensity of different portions of the nebula to be measured. Similar 

 results were obtained on Plates 368, 372, 374, and 382. 



The large angular aperture of the lens employed is especially advantageous in 

 photographing the fainter portions of this nebula. An exposure of about 5'" gives 

 the best image of the central portions of the nebula. With 15"" the central portion 

 is completely burned out, and the nebula has as great an extent as is shown in the 

 beautiful photograph obtained by Mr. Common on Januar}^ 30, 1883. This photo- 

 graph was taken with the 36-inch reflector, and had an exposure of 39 minutes. The 

 images of the stars in Mr. Common's photograph, owing to the greater focal length 

 of the telescope, are much smaller and better defined than in the Cambridge photo- 

 graphs. The latter, vnih an exposure of an hour, gives an extension to the nebula 

 of about a square degree. The nebulosity around c Ononis is shown with much detail 

 in these photographs. 



The nebulositj' around the star Maia in the Pleiades, discovered by MM. Henry, 

 was confirmed by Plate 104, taken on November 3, 1885. Although this nebulosity 

 was at once recognized, it was ascribed to a defect in the plate until the true 

 explanation was given by MM. Henry. The first photograph of this object by these 

 gentlemen was taken thirteen days later, or on November 16, 1885. 



Jupiter's satellites, as might be expected from their briglitness, gave well-marked 

 trails when the telescope was at rest. Some experiments were accordingly made to 

 determine whether the times of the eclipses of the satellites could be advantageously 

 observed by photography. Were it not for the presence of the planet, this would 

 probably be the most accurate method of determining these times, employing the 

 method described on page 211. When the telescope was driven by clockwork, good 

 images of the satellites were obtained in two seconds, and the images were over- 

 exposed when the exposure much exceeded ten seconds. During an eclipse the 

 experiment was tried of making a series of exposures, each lasting for ten .seconds, 

 at regular intervals of fifteen seconds. The telescope was moved a short distance 

 in right ascension or declination after each exposure, and during the five seconds 

 preceding the usual exposure. Excellent images of the more remote satellites were 

 obtained, but, owing to the short focal length of the telescope, the image of the 

 satellite undergoing eclipse was obscured by that of Jupiter, which was large, on 

 account of the length of the exposure. 



