214 JOURNAL OF SCIENCE. 



minutes' exposure records stars of the 11th magnitude, then 30 to 35 

 minutes should record those of 1 4th ; but here in the case of a well 

 known cluster, with every star recorded by a careful observer, it is 

 found that the rule fails, and the question arises, Did he over-estimate 

 these stars or did the rule fail ? Over nearly the whole surface of the 

 sky we have no record of stars below the 9 th magnitude, and therefore 

 no means of finding whether the photographs do really record what 

 is desired, that is, stars of 14th magnitude, and it is obvious that 

 more experiments will have to be made upon well-known clusters, and 

 thus determine the time necessary for the purpose of making certain 

 of 14th magnitude stars; when that is done, however, we shall have 

 in the photographs a vast number of stars of the 14th magnitude 

 which the eye cannot see through the telescope, just as I found in 

 Kappa cruris. The extended exposure in order to secure visible 14th 

 magnitude stars ended in recording a large number of stars photo- 

 graphically as bright as them, but wholly invisible through the 

 telescope. At the recent meeting of the committee it was decided, 

 on the evidence given by Dr. Scheiner, to extend the time of exposure 

 to 40 minutes, and it is reasonable to expect that, since all are 

 interested and working at this point, it will soon be decided, and 

 times of exposure agreed upon for different states of the atmosphere 

 which will ensure uniformity. At present there seeme to be no 

 possibility of dealing with the colour difficulty, which is a serious one, 

 as I have already pointed out. Great differences are found also in 

 the sensitive plates. We have tried Swan's, Wrotton and Wain- 

 wright's, Field Dodgson's star plates, M.A. seed plate (American), 

 and Ilford plates ; and the Ilford plates are certainty the best for our 

 purpose. The American plates are, perhaps, less liable to fog, and 

 work very cleanly and regularly ; but they are not so sensitive, and 

 the gelatine is not so firm. Again, in my photos of the great 

 Magellan cloud, taken with the portrait camera which I exhibited at 

 the November meeting, the stars, owing to their countless numbers, 

 are condensed into a blurred mass, and the great and remarkable 

 nebula 30 Doradus is only a white spot. With the star camera the 

 picture is all enlarged ten times, and the stars are separated and 

 brought out sharply defined, while the nebula 30 Doradus is revealed 

 in all its wonderful complexity, and shown to be much more extensive 

 than Herschel made it with his great reflector, and quite a new light 

 is brought out regarding the structure of this object. There is one 

 thing about this nebula that is very suggestive. Some of its loops 

 are quite round, and all its features seem to be laid out as if in a 

 plane at right angles to the line of sight. There is no sign of 

 elliptical forms, which so commonly appear in nebula?, owing to their 

 circular forms being oblique to the line of sight, and therefore pro- 

 jected into ellipses. If we look at the main features of Nubecula 

 major the same remark is applicable : the curves are nearly circles, 

 both those in the main body of it and in the several star clusters and 

 nebula ; they all, in fact, seem to lie at right angles, or nearly so, 

 to the line of sight. Now, just as the sun with his attendant planets, 

 and the planets with their moons, and especially Saturn, with his 

 rings, show us that there has been a tendency, as theory would also 

 lead us to expect, to arrange the matter that is revolving about them 



