170 Mr. David Gill [June 3, 



star discs remain well defined. The corresponding region of the sky 

 is less than the moon's aj^parent diameter, and of the many thousands 

 of stars visible on the photograph not a single one is visible to the 

 naked eye. The star rj Argus was in 1843 nearly the brightest star 

 in the heavens ; in fact, second only to Sirius. It is now between 

 the 7th and 8th magnitude. 



Here is a star-cluster in Argus. The star discs are not so sharply 

 defined, but the original negative has been much magnified to bring 

 out the star discs. 



Here is a photograph, also taken at the Cape, of the wonderful 

 star cluster w Centauri. It is the finest globular cluster in the 

 heavens, and I do not know that I have ever seen the sej)aration of 

 the central stars so distinctly with the eye as they are shown in this 

 photograph. Perhaps by photographing we shall learn what motions 

 occur in each cluster. This negative has been enlarged four dia- 

 meters from the original. 



Here is a photograph of the well-known cluster in Hercules, 

 taken by Mr. Eoberts, of Liverpool, and a still more wonderful one 

 by the Henrys, of Paris. They must be magnified more highly to 

 give any idea of their quality. 



When the objects are bright, such as bright double stars, or 

 planets, or the moon, we can enlarge the image produced by the 

 telescoiDC, by aid of a secondary magnifier. 



Because of the greater size of the original pictures thus produced, 

 the granulation of the photographic film interferes to a less extent 

 with the detail of the picture. Of course, this advantage is pur- 

 chased at the cost of a longer exposure, because the same amount of 

 light is spread over a larger area of the sensitive plate, and con- 

 sequently the same area of the film receives less intense light. With 

 very bright objects, such as the sun, moon, and planets, this is of 

 little consequence, and may be an advantage, as permitting more 

 accurate regulation of the exposure. 



Here is a picture of the sun photographed by M. Janssen at 

 Meudon, near Paris. The exposure is less than 1/lOOOth of a second 

 of time. And here is an enlarged photograph of the same spot, 

 showing an amount of detail which no artist could convey by hand 

 and eye, nor could he emulate the absolute accuracy of the photo- 

 graph. 



Here are some photographs of the planet Jupiter, taken at Paris, 

 the original image being magnified 18 times. 



Here is another showdng the remarkable red S23ot — you even have 

 before your eyes evidence of the rotation of Jupiter on its axis by the 

 change in the position of the spot during the same evening. 



This spot appeared in 1878 and measured about 30,000 miles in 

 length by 7000 miles in breadth. It became of a deep red colour in 

 1879, and for the three following years was a most striking feature 

 in the planet. It almost faded entirely in 1883, but has again 

 become nearly as bright as it was in 1882. 



