108 H. C. EUSSELL.— ON SOME PHOTOGRAPHS OF THE 



and it is so unlike a picture of stars that I have only brought one, 

 just to show you what it is like; the best way I have so far found 

 of showing the stars is to make a glass positive from the negative, 

 and this with a bright light behind gives a good idea of a field of 

 stars, and it is very striking to see how the fainter stars shew 

 out in this process, provided the photo manipulation is carefully 

 done. To what extent it would be possible to carry the exposure 

 is doubtful ; pictures of interiors of buildings have, I believe, been 

 taken which required ten days' continuous exposure so faint was 

 the light, and there seems to be no reason why plates should not 

 be exposed on the same object night after night until even the 

 faintest objects recorded themselves, if due care was exercised 

 in covering the plate during the day and in other necessary 

 precautions. I have experimented in this direction, that is as to 

 the best time for my purpose to expose the plates, and have come 

 to the conclusion that three hours on a fine night is enough for a 

 first series. 



Subsequently another set may be taken with six hours' exposure 

 in order to bring out still fainter stars, but the present series will 

 show stars to the loth magnitude. Owing to weather difficulties 

 and the short time the lens has been available I have only got good 

 pictures of the Milky- Way about Eta Argus, Alpha Centauri, the 

 Southern Cross, and a point near Alpha Centauri to shew you. 

 Each of these cover a considerable surface, one hour in R.A. and 

 fourteen degrees in declination. If you look at the map you will 

 see that five such pictures would take in all the sky over which 

 the Milky Way is supposed to extend, from the great break near 

 Eta Argus to the constellation Lupus, or nearly all of it that is 

 near the South Pole. In many respects the most striking of the 

 photos I have to show you to-night was one taken on the 24th 

 of July last with the star Alpha Centauri in the centre. The 

 night was fine but not first-class, and the plate was exposed for 

 two hours fifty-five minutes and it seems literally covered with 

 stars. 



In the densest part near the centre I have counted in one square 

 degree 1,108 stars, taking next the part which seemed to have 

 fewest stars I found 663 to a square degree ; I have counted a 

 number of other portions and get an average of 862 stars to a 

 square degree. As I have already explained, it is only the central 

 part of the plate, a portion containing nine degrees on each side 

 or eighty-one degrees where the definition is good, and over this 

 space then we have 862 stars in each square degree or 70,000 

 stars ; from counting other portions outside this limit of good 

 definition I estimate that there are another 30,000 stars, or a 

 total of 100,000. Sir John Herschel's star gauges of this part of 

 the sky give 936 stars to the square degree, and his telescope was 



