92 



NATURE 



[November 24, 1892 



The composition of the Youndegin iron was found to be as 

 follows : — 



Iron 92'67 



Nickel 6-46 



Cobalt 0-55 



Copper 



Magnesium 



Phosphorus ... 

 Sulphur 

 Insoluble cubes 



trace 



0'42 



o"24 

 none 

 o'04 



100 38 

 James R. Gregory. 



THE CROSS-STRIPING OF MUSCLE. 



pROF. RICHARD EWALD of Strassburg, has just com- 

 municated a paper to the fifty-second volume of the Archiv. 

 J. d. ges. Physiol., in which he confirms Prof Haycraft's views 

 concerning the structure of striped muscle. The latter ob- 

 server has held for many years that muscle fibrils are varicose 

 threads, and that the cross-striping is but an optical appearance 

 due to this varicosity. The varicosity is often difficult to demon- 

 strate in the ordinary way, and most histologists were not pre- 

 pared to admit that the stripings are all and entirely due to it. 

 Prof. Haycraft, however, recently brought forward to the Royal 

 Society of London, and to the Berlin International Medical 

 Congress, fresh and striking proof of the strength of his position, 

 by demonstrating films of moist collodion, on which pieces of 

 muscle had been pressed and then withdrawn. As a result of 

 this pressure the collodion films were stamped as with a seal, 

 and the impressions revealed in striking detail every stripe of 

 the original fibre. Prof Ewald confirms these experiments in 

 the fullest manner, but suggests that the collodion impressions 

 might be produced on the assumption that there are layers of 

 hard and soft material alternating with each other in the course 

 of the fibrils. In this case the hard material would press into 

 the collodion and make a series of furrows, which would appear 

 as a series of stripes when examined with the microscope. Prof 

 Haycraft had previously demonstrated the varicosity of the fibrils, 

 seen by transmitted light, and had published photographs of his 

 preparations, but Prof. Ewald was still sceptical upon this par- 

 ticular point, and sought to assure himselfstill more conclusively. 

 With this end in view he examined muscle, which had been 

 rendered quite opaque, by means of reflected light, for under 

 these circumstances the influence of the internal .structure would 

 •be entirely set on one side, and the surface of the fibrils would 

 alone receive and reflect the illuminating rays. For purposes of 

 illumination Prof Ewald used the apparatus of W. and H. 

 Seibert, of Wetzler, by means of which vertical rays can be 

 projected upon an opaque object ; and he rendered his prepar- 

 ations, both of fresh and of hardened muscle, quite opaque by a 

 method of over-silvering. Under these conditions Prof Ewald 

 'found that the cross-striping is most distinct, and he was able, 

 with his admirable method of illumination, to examine the sur- 

 face of a muscle just as one may observe the surface of the 

 country at night by means of a search-light from an observatory. 

 With the light perfectly vertical the tops of the ridges of the 

 muscle are bright, and the valleys on either side in half-light. 

 By shifting the light to one side or to another the slopes of the 

 ridges can be thrown alternately into shadow or bright light. 

 Prof. Ewald concludes by admitting that his experiments fully 

 rprove that the striping is due to the shape of the fibrils alone, 

 and that the internal structure of the muscle plays no part in its 

 production. 



IRIDESCENT COLOURS} 



ON taking a general survey of coloured objects, whether 

 natural or artificial, we become aware of the fact that 

 whilst the colours of some remain unchanged as regards tint, 

 whatever their position in relation to the incident light, the 

 ■tint of others varies with every alteration in their relationship 

 to such light source. We thus see that so far as their colours 



1 By Alex. Hodgkins^n, M.B., B.Sc. Reprintedfromthefifth volume of the 

 ■fourth series of " Memoirs and Proceedings of the Manchester Literary and 

 -Hhiloiophical Society." Session 1891-92. 



NO. 1204. VOL. 47] 



are concerned all bodies may be arranged in two groupS 

 according as their colours change or do not change in tint as 

 their angular relationship to the light varies. Nor is this 

 classification entirely an artificial one, since, as will shortly be 

 seen, though this change in tint with variation in the light 

 source is an essential difference, it is not the only difference, 

 even in the colour manifestations of the two groups, for it is 

 also characteristic of the nature of the colour-producing structure. 

 It is to the above-mentioned varying colours that we apply the 

 term iridescent, from the resemblance they have in the sequence 

 or play of colours to the tints of the rainbow. The unvarying 

 group of colours, having no equivalent term to "iridescence " to 

 express the nature of their colour produc tion, are spoken of 

 as " pigmentary," or absorption colours. In naming examples 

 of objects, natural and artificial, grouped as above in accordance 

 with the nature of their colours, it is difficult to make a selection 

 where all are so varied and characteristic. I have preferred 

 therefore to cite only such instances as I myself possess, and 

 am therefore able to show you. As examples of pigmentary 

 colours, I need only name one or two for the sake of comparison, 

 since the colours of most objects ordinarily met with are pig- 

 mentary. Leaves, flowers, dyes, birds, fish, insects, minerals, 

 &c., exhibit these colours, some almost entirely, and all, 

 excepting fish, in far the majority of instances. Of objects 

 displaying iridescent colours we have also examples in the 

 various divisions of the animal, vegetable, and mineral 

 kingdoms. Amongst birds the most striking examples are 

 found amongst the humming birds, sun birds, birds of 

 paradise, &c. Insects, again, furnish numerous examples, 

 more especially amongst tropical species, though not, perhaps 

 proportionally in greater numbers than amongst those belonging 

 to our own more temperate regions. The colours of fish are 

 almost entirely iridescent, since their very whiteness, or silvery 

 sheen, is due to the admixture of the iridescent colours of 

 innumerable minute thin lamellae, too small to be seen individu- 

 ally with the naked eye, but plainly perceptible under the 

 microscope. In the vegetable kingdom iridescent colours are 

 far more numerous than is ordinarily recognized, since 

 the surfaces of the cell walls produce interference colours which 

 are more or less obscured by the pigmentary colours of leaves 

 and coloured flowers, but may be readily seen in the case of 

 white flowers by the aid of a lens and sunlight. Under these 

 conditions each cell may be seen to sparkle with its own 

 iridescent colour, forming, by admixture of the interference 

 tints of neighbouring cells, the varying shades of white seen in 

 numerous flowers which are devoid of pigmentary colour. 

 Mineral bodies displaying iridescent colours are also numerous ; 

 opals, sunstone, fire-marble, felspar, mica films, tarnish on 

 various metallic crystals, certain crystals of chlorate of potash, 

 &c., are examples. 



In describing the various natural objects for purposes of 

 identification, or mere description, no account can be considered 

 complete which omits ail reference to their colours, and mote 

 especially is this the case where the colours constitute such a 

 striking feature, as in the case of iridescent bodies. In innum- 

 erable instances, more especially amongst birds and insects, 

 their specific names are taken from some conspicuous colour 

 they possess. It thus becomes evident that a correct descrip- 

 tion of the colours of bodies is of importance, and where these 

 colours are of the pigmentary, or unchanging kind, this is a 

 matter of no difficulty. How different, however, in the case 

 of objects, the colours of which not only vary with every change 

 of position, but disappear altogether, unless viewed with special 

 relation to the light source. Nor can it be wondered at that 

 descriptions of these objects, even by observers of undoubted 

 repute, vary according to the diff'erent angles from which they 

 have been viewed ; or are vague and profuse, owing to fruitless 

 attempts to describe their changing tints produced by every 

 movement. The fact is, no words can convey an adequate 

 impression of the gorgeous effects produced by most of such 

 objects, whether birds, insects, or fish, when in motion in 

 brilliant sunshine. Some notion of the' difficulties to contend 

 with in describing the colours of humming birds, for example, 

 may be gathered from the remarks of Wallace in his work on 

 "Tropical Nature," when speaking of humming birds: — ''In 

 some species they must be looked at from above, in others from 

 below ; in some from the front, in others from behind, in order 

 to catch the full glow of the metallic lustre ; hence, when the 

 birds are seen in their native haunts, the colours come and i;o 

 and change with their motion, so as to produce a startling and 



