414 Professor H. H. Turner [Jan. 31, 



the compoiients. Their masses are closely similar, like those of 

 pebbles on a beach. But this tells us nothing about the sizes of the 

 minute discs. Have I gone too far peihaps in saying there is no disc 

 visible in any star ? Nebulae do show discs, and though they are 

 nebulae and not stars they may become stars. Here is a beautiful 

 picture of the "Owl" nebula, taken with the Mount Wilson five-foot. 

 It shows a beautiful circular disc with a star in the centre. Com- 

 paring Lord Rosse's drawing of it made with his wonderful six-foot 

 reflector, we see the much greater perfection of the photograph ; but 

 this is chiefly due not to the inferiority of Lord Rosse's instrument, 

 which was a marvel of engineering skill, but to the greater efficiency 

 of photography compared witli the eye. The new secrets w^rested 

 from the stars have chiefly come, not from the increase in size of 

 telescopes, but from the new appliances attached to them, such as the 

 photographic plate, the spectroscope, and by this time many others. 

 The lines in the spectra of stars tell us what the stars are made of, 

 how they may be classified accordingly, how fast they are moving, 

 how bright they really are (this is an amazing recent discovery), and 

 by inference how far away, and may yet have other surprises in store. 

 For the moment we are chiefly concerned with the classificafion. 

 The Harvard system gives us a number of classes denoted bv the 

 capital letters O" B A F G K M R N. The fact that the order is not 

 quite the same as that of the alphabet represents a revision of early 

 ideas, chiefly due to the gradual accumulation of intermediate types, 

 which make a nearly continuous series. 



Now a series of stars in order is probably a representation of 

 growth ; just as the growth of trees may be illustrated by selecting 

 various stages from the same wood, an illustration originally given 

 by Sir W. Herschel. But we have seen a tree grow, and we know 

 independently that it grows up from the acorn through the sapling 

 to the giant oak ; while we have not had time to see a star grow and 

 were thus in ignorance whether the changes are from B towards M 

 or from M towards B, though by this time we have an immense 

 number classified. The classification has been largely the work of an 

 American lady, Miss Cannon. I am told that there is a man who 

 can deftly straighten rifle barrels — he gives a glance along the barrel, 

 a tap with a hammer, and lo ! it is straight. His value is recognised 

 at some £15 a week. Miss Cannon has the same deftness with 

 spectra — but I fear that (to judge from the report of the Board of 

 Visitors of Harvard Observatory) her great skill is not so appro- 

 priately rewarded. 



Now it is obviously important to find out, if we can, which is 

 the direction of a star's growth, and we seemed to have an important 

 clue when the spectral classification was connected with the tempera- 

 ture of a star, or rather its surface temperature, which is all we can 

 get at. The outside is the coolest, just as the edges of a plate of 

 porridge are the coolest, as most of us have learnt by early and rather 



