THE PHOTOGRAPHIC CHART OF THE HEAVENS. 63 



object glass of the Star Camera. A small electric lamp of two 

 and a-half candles is then put in the focal point of the Star Camera, 

 and the rays from it pass out from the object glass parallel, and 

 falling on the rhf.au are all stopped by the silver, except those 

 which fall on the lines and figures, and these pass through on to 

 the sensitive plate and mark it. A number of plates are so 

 treated one after the other and stored ready for use in dark 

 boxes. They are all carefully numbered on the glass, and exposed 

 in the Star Camera in order of the numbers. The plates measure 

 6i X 6^ inches, and the part actually exposed 6x6 inch; of this 

 space 4*7 x 4*7 inch is the portion which is finally used, that is 

 2° x 2°. Of the margin, rather more than half inch serves as 

 overlap on the plates, and the stars on this can be compared for 

 verification in each adjoining pair of plates. When the plate is 

 developed after exposure, the lines or grating as well as the 

 stars appear. 



So far everything is simple and mechanical, but the resolutions 

 of the Conference require, that one set of plates shall have on 

 them all stars to the 11th magnitude, and the other set all stars 

 to the 14th magnitude, and the difficulty in an everchanging 

 atmosphere, and with plates which differ in sensitiveness, is to give 

 the exposure necessary to secure these results. 



The Astronomer Royal of England as Chairman of the Com- 

 mittee appointed to deal with these and other kindred questions, 

 has been making experiments on a fairly good night in London. 

 He has come to the conclusion that two minutes will be enough 

 in such weather for stars of 11th magnitude, and thirty minutes 

 enough for stars of 14th magnitude, and that these times must be 

 varied to suit the weather, that is increased if the weather is bad. 

 I am able to shew you three plates, one exposed thirty seconds, 

 another two minutes, and the third thirty minutes, on the well 

 known star cluster Kappa Crucis. You will see that thirty 

 seconds is enough to get images of stars to the 9th magnitude, 

 and that two minutes gives images of stars to the 11th magni- 

 tude, and takes in also some of 12th and one of 13th magnitudes. 



