April 1, 1897.] 



KNOWLEDGE. 



99 



We have, then, the case of the "fifth type" spectra, 

 distributed, as is so well known, practically along the 

 galactic equator. Is it possible to conceive that we have 

 one well-marked temperature stage confined to a single 

 great circle of the heavens ? 



There is another point. A small star will undoubtedly 

 " age " more rapidly than a large one. Consequently, 

 when we have in the same group, and, roughly speaking, 

 at the same general distance from us, stars of very wide 

 range of magnitude, we have the right to expect, on Mr. 

 Fowler's assumption, a very wide range of spectral type. 

 Indeed, in such groups, we might reasonably look for the 

 entire series. The Pleiades is a case in point, and here 

 we find practically but a single type. More striking still, 

 in the Milky Way — the components of which, as the 

 arguments of Proctor, Eanyard, and Sutton have shown, 

 are relatively of no very wide range of distance from us, and 

 where we find with an enormous range of stellar magnitude 

 an immense clustering of very small stars — we have a 

 predominance of the Siriau type. It is just here in the 

 association of large and small stars of the same type in the 

 same cluster that the difficulty consists. We ought to 

 find in such cases the usual evidence of difference of 

 temperature condition, but we do not. 



Further, Mr. Monck has shown that there is a marked 

 correlation between the amount of proper motion and 

 spectral type. It is dJfticult to see how temperature and 

 proper motion are connected. 



In conclusion, we must suppose, on Mr. Fowler's 

 assumption, that nebuhv are, like stars, all of the same 

 proportionate composition, and are all. strictb/ homo;/encous 

 throui/Ziout. 1 merely suggest this point for Mr. Fowler's 

 consideration. — E. Walter Maunder.] 



STAR SYSTEMS. 

 To the Editors of Knowledge. 



Sirs, — Mr. Monck's letter, page 72, is an important 

 addition to bis letter in Vol. XIX., p. 274, laying down, as 

 it does, the three conditions which he considers necessary 

 before conmoa proper motion can be held to be evidence of 

 physical connection. The two last of his conditions will, 

 I think, be geaerally accepted, and Mr. Monck's statement 

 that they are not fulfilled in the case of the stars in the 

 Plough is also an important addition to the discussion. 



It would, however, be more satisfactory if he would 

 furnish the exact proofs upon which he bases this conclusion, 

 for Proctor's statement with reference to the second con- 

 dition, " that the correspondence in their motions should 

 be close," is that such is the case with regard to the five 

 stars ,3 to ^ their apparent motion being unusual and 

 contrary to that of the majority of bright stars. The 

 drawing on page 2.51 of your Vol. XIX. shows that Proctor 

 is correct with respect to the stars y, o, s, ^; but he also 

 adds the two stars Alcor, and the one between it and ?, as 

 having nearly the same motion. This would make six 

 stars within a space of about twelve degi-ees with the same 

 drift, thus fulfilling the first condition laid down by ^Nlr. 

 Monck, " that the stars be situated within a moderate 

 distance of each other." It is a question, however, whether 

 with our limited knowledge this should be considered a 

 condition, or what the expression, " a moderate distance,' 

 can be held to mean. It may be easy to believe that stars 

 at a much greater apparent, and even actual, distance apart 

 than these stars may have a common drift and a common 

 origin. 



As regards the third condition, " that the motions them- 

 selves be of such a character that only a small proportion 

 of them can be ascribed to errors of observation," Mr. 



Monck might suitably supply the grounds upon which he 

 believes that the condition is not fulfilled in the case of 

 the Plough. T. W. Backhouse. 



Sunderland, ?ilarch 17th, 1897. 



THE LEAF-CUTTER BEE. 

 To the Editors oj Knowledge. 



Sirs, — I am sorry if Mr. Enock considered my reply to 

 his letter in the January Number of Knowledge in any 

 way discourteous, but I would recall to his lemembrance 

 that it was he in the first instance who accused me of 

 inaccuracy, and I was in justice to myself bound to reply 

 to his challenge and prove my facts, if I did not choose 

 calmly to submit to Mr. Enock's inference that I had 

 written upon a subject of which I knew little or nothing. 



Had Mr. Enock read my letter more carefully, he would 

 have seen that I did not state that I relied solely upon the 

 observations of " well-known scientific men." I have 

 always been in the habit of investigating and proving facts 

 for myself, since the time when I worked under the guidance 

 of my friends and teachers the late Charles Darwin and 

 Prof. Huxley ; but I do not altogether ignore and discredit 

 the works of acknowledged authorities, and consider my 

 own observations and opinions as superior to those of 

 anyone else. 



Mr. Enock's photograph is certainly remarkable, and it 

 may interest him and others of your readers to see one 

 that I have taken of a cell of the leaf-cutter bee (Meimfhile 

 centuvctdaris), the insect I described in the November 

 Number of Knowledge. The folding under of the leaves 

 to form the base of the cell may be seen in Figs. 1, 4, 5, 

 and 6. 



Photograph oE Pieces of Leaves forming a Cell of Megachile 

 centuncidaris. Big. 1. — First or Outer Sheath, showing the lower 

 portion folded under to form the Base of the Cell. Fig. 2.— Inuer 

 or First Cap, closing the Coll. Fig. i. — Second or Outer Cap, placed 

 above First Cap. Fig. 4. — Second Sheaths. Fig. 5. — Third Sheaths. 

 Fig. 6. — Inner Leaf. 



I quite agree with Mr. Enock that the pudding when 

 made by mother bee is sweet and fragrant, but there is an 

 old saying that "the proof of the pudding is in the eating," 

 and by the time the larva tastes it it has become as I 

 described. 



I forgot to mention in my last letter that there are other 

 leaf-cutter bees who construct their cells with pieces cut 

 from the leaves of the willow and the petals of the poppy. 

 Thanking you for the courtesy you have shown in publishing 

 my replies to Mr. Enock's letters, 



Francis M. Dunc.\n. 



1, Lincol Villas, Gloucester Road, Redhill, 

 March 1.5th, 1897. 



