226 



KNOWLEDGE 



[December 1, 1892 



large area of the Milky Way, and divide all the stars we are 

 able to detect in it with a given power, into half a dozen 

 classes of an ascending order of magnitude, containing, 

 say, o, h, c, d, e, f stars respectively in each class, then the 

 ratio a : b : c : d : i: f, is nearly enough constant for the 

 same magnitudes over the whole ]\Iilky Way. 



We have, then, some reason for the inference that we 

 may reason upon the Milky Way stars as upon an equal 

 number of stars of a standard size equal to the average i 

 magnitude of the true Milky Way stars. In short, if the ' 

 Milky Way have a stars giving /* units of light each, h stars > 

 giving I] units of light each, c stars giving r imits of light ', 

 each, &c., we may for present purposes regard it as con- 

 sisting of a + b4-c+ . . . equal stars each shining with 

 ( ap+bq-i-e.-+ .) ^^jjjjg q£ J- JjJ g^jjj jj^jg fraction may be 



( a+b+c+ ) o ' J 



regarded as the standard star for any galactic area 

 of considerable extent, leaving the lucid stars, of course, 

 out of consideration. ■ May we, therefore, argue with equal 

 confidence for an equality of distribution ? Clearly we 

 may not, unless we are to adopt the unscientific conclusion 

 to which the first calculation in this paper would lead us 

 that exactly where the MOky Way is brightest there it is 

 farthest away, and that its breadth increases in exactly 

 the same proportion. The only apparent alternative is 

 that the greater brilliancy of the Milky Way in parts of 

 Cygnus and Scorpio, and in the great bright galactic clouds 

 of the southern hemisphere, is due to a greater number of 

 stars in a given volume. 



Such evidence as it is amounts to (1) a general circularity 

 of section ; (2) a general equality of magnitude ; and (3) 

 an unequal distribution of the stars throughout the Milky 

 Way. We can state, therefore, as at least an approximate i 

 truth, that the brightness of any part of the Milky Way is { 

 a measure of the relative number of its constituent stars, 

 and, in a less degree, of its relative mass. Now the Milky 

 Way in the southern hemisphere is of considerably greater 

 brilliancy than it is in the northern hemisphere ; or, to be 

 more correct, the portion of the stream reaching from 

 Cygnus through Aquilla to Argo, is brighter than the 

 remaining portion, an arrangement followed, strangely 

 enough, by the belt of great stars which intersects the 

 MOky Way in Cygnus and Argo. It is in the southern 

 hemisphere, then, that the matter of which the Milky Way 

 is composed should be found in the greatest profusion. 



The actual proof or disproof of this purely speculative 

 result, however, can only be gradually evolved by years of 

 work in the observatory. There are two chief difficulties 

 to face : one the difference of texture, so to speak, between 

 the northern Milky Way and the southern ; the other, the 

 possibility that the brightness of the galactic clouds may 

 be due in part to local collections of clashing meteoric 

 matter, though it must be admitted that meteoric matter is 

 on the whole quite as likely to stop light as to send it. 

 We can at any rate assert with some confidence that the 

 brighter the region the greater the number of stellar points 

 comprised within it, whether our instruments reveal them 

 individually or not. 



[Mr. Sutton leaves out of account brightness due to 

 nebulosity between the stars. — A. C. Kanyard.] 



* I assume here that the sun is not far from tlic centre of the 

 galactic ring. The aspect of the Taurus-Orion belt of great stars 

 seems to indicate that the true Milky W.ay stars are limited to tele- 

 scope magnitudes. Nearly every bright star on the Milky "Way 

 appears to belong in some way to this belt, and the galaxy itself is 

 rich in bright stars because it meets the belt at a rery acute angle. 

 This consideration eliminates the lucid stars from consideration here, 

 and hence reduces the number of magnitudes we have to deal with. 

 The exactitude with which the standard star represents the true 

 average star is correspondingly increased. — .J. R. SriTOir. 



Fig. 3. — ^Leg of Caterpillar 

 of Groat Motli. magnifiecl 

 8 diameters. 



CATERPILLARS.-II. 



By E. A. Butler. 



[Continued fro^n paije 206.) 



CONTINUING the account of caterpillar structure 

 which we commenced last month, we come to the 

 three segments immediately behind the head. Each 

 of these carries a pair of conical legs terminating 

 in a sharp curved claw (Fig. 3). From their situa- 

 tion it is evident they correspond to the legs of the perfect 

 insect, and that the three segments to which they are 

 attached represent the thorax of the butterfly or moth, 

 although there is here no definite 

 line of division, as there would be 

 in the perfect insect, to separate 

 them from the rest of the body. 

 These legs are of considerable use 

 to the caterpillar, not only in walk- 

 ing, but also and more especially 

 in holding the leaves while it is 

 feeding. The blade of the leaf, 

 being attacked at its edge, is steadied between the 

 caterpillar's legs, while its head is stretched out to the 

 full, and its mandibles, having been opened laterally, are 

 brought with a sudden snap down upon the leaf edge lying 

 between them. By this action a fragment is punched out 

 as it were and immediately swallowed, while in far less 

 time than it takes to describe the action, the head is bent 

 round along the arc of a circle and a fresh snip taken as it 

 is brought into each new position, so that in the end a 

 curved excavation is made in the tissues of the leaf. 



Throughout the whole tribe of caterpillars there are 

 scarcely any other organs that are so uniform in structure 

 as these legs, and there is rarely anything exceptional about 

 them, however varied may be the habits of the insect. But 

 we have one species, the caterpillar of the lobster moth 

 [Suiiiropm fii;ii), which deviates notably from this uni- 

 formity — a deviation sufficiently remarkable to lead us to 

 suspect and look about for some adequate cause. In this 

 rare and curious caterpillar two of the three pairs of legs 

 are greatly elongated and, pointing forwards, sprawl out at 

 the sides in a rather spider-like fashion ; at the same time 

 the front pair, which are not elongated, are widely opened 

 somewhat as the jaws of a spider might be made to gape, 

 and the hinder part of the body is bent up over the back, 

 thus making some sUght approximation to the shape of a 

 spider's globose hind parts. Now as this is not the ordinary 

 attitude of the caterpillar, but is only assumed when it is 

 irritated, we may reasonably conclude that its assumption 

 is intended as a means of defence, and it has therefore 

 been supposed that this " terrifying attitude," as it is called, 

 suggests to the creature's enemies a dangerous foe of the 

 spider class and induces them not to molest it. It is true 

 that the resemblance to a spider is far from perfect ; in 

 particular, the body is not so broad as a spider's would 

 be, and there are only two pairs of legs extended instead of 

 four ; in fact, as Mr. Poulton, who has had good 

 opportunities of watching this larva, admits, the " sug- 

 gested monster" is " exactly like nothing upon earth, but 

 is, nevertheless, most effective in its appeal to the 

 imagination." At any rate, to an enemy gifted with some 

 degree of imaginativeness, and approaching it in front, 

 it would look sufficiently uncanny to suggest the idea of 

 possible powers of harm-doing, and thus rouse feelings of 

 suspicion and cause a would-be captor to hesitate before 

 attacking it. Of course, if the enemy is shrewd enough 

 to see through the deception, the fate of the larva is sealed, 

 for it is in no way able to inflict injury or to protect itself 



