Atigrist 25, 1887] 



NATURE 



391 





of the ventral surface of the oesophagus there is to be 

 found a group of large ganglion cells containing extremely 

 large nuclei with viscid protoplasm and prolonged fila- 

 ments. Mention is also made of the grooves lined with 

 long flagelliform cells, which, however, were some time 

 since described by Hickson in a paper published in 

 the Philosophical Transactions under the name of 

 *' Siphonoglyphe." 



Another novel phenomenon was observed in a species 

 of Nephthya, where several of the polyps seemed to be 

 solely reproductive, and in them as soon as fertilization 

 was effected, the tentacles became incurved over the oral 

 aperture, which then became plugged with a viscid 

 mucous, and apparently during the gravid period these 

 polyps were nourished by the other polyps of the 

 colony. 



We must content ourselves with giving but a very brief 

 summary of the forms described. The genus Voeringia 

 is established for a series of branched Alcyonids with 

 retractile polyps, in this differing from those of Duva ; 

 eight new species of this genus are described, to which 

 also the Alcyonium fruticosum^ Sars, is referred. Eight 

 new species of the beautiful genus Duva are recorded. 

 A new genus, Drifa, is established for an arborescent 

 cies, the spicules in which differ from those of both 

 ringia and Duva ; of the two species, one, D. I'slatidica, 



hibits an interesting structure ; around the mouth and 



tween its external opening and the base of the 

 tentacles, there are eight little fringe-like protuberances, 

 which form a ruff. An appearance of the same kind, only 

 outside the circle of the tentacles, we have observed in 

 a Plexaurid, but we are not certain but that it may be due 

 to the sudden immersion of the polyps into strong spirits. 

 For a graceful arborescent form with auto- and siphono- 

 zooids, which reminds us of Anthomastus, Verrill, the genus 

 Nannodendron is proposed ; the polyps are completely 

 retractile. Fuila schiertzi is a new genus and species of 

 another branching form with a somewhat flattened stem, 

 showing a distinct bilateral symmetry, the branches only 

 springing from the opposite sides of the main axis. Three 

 new species of Nephthya are enumerated. For a species 

 in which in addition to a well-marked siphonoglyphe 

 there are also in the first part of the oesophagus two 

 flap-like protuberances, the genus Gersemiopsis is made. 

 The only species, G. arctica, was dredged in a depth of 

 658 fathoms. A new genus, Barathrobius, is made for 

 two new species, in which the basal part of the colony is 

 hard and often dilated, the polyps are retractile, appear- 

 ing only, when fully withdrawn, as slight elevations above 

 the mass of the branches. Sarakka crnssa (n g. et sp.) is 

 a species with a very peculiar structure in its oesophagus, 

 which seems to be constricted laterally into two indepen- 

 dent portions ; while Crystallofancs polaris is a form with 

 few polyps on the stem but with a summit rich in polyps, 

 borne on short branches which are placed in whorls 

 round the stem ; the polyps are retractile. 



A new sub-family is made for a new genus and species 

 Organidits nordenskjoldi ; in this species the polyp cells 

 are long, connected together so as to form an axis ; these 

 polyp cells are long, cylindrical, calcareous, with both the 

 polyp body and its tentacles well provided with spicules. 

 The author thinks that this sub-family shows some affinity 

 to the TubiporiniC, but it would appear to us to show 

 more relationship to such forms as Gersemia and 

 Eunephthya. Clavularia frigida and Syinpodiuin 

 abyssorum are described as new species. 



This memoir is published in both Swedish and English, 

 in parallel columns, for which the student cannot be too 

 thankful ; true, the English may strike the reader as a 

 little quaint, and in the nomenclature of the spicules it is 

 somewhat novel, but criticism would be out of place in 

 the presence of so great a boon. The day is coming 

 when a new classification of the spicules of the Alcyonaria 

 must be made ; at present, while new types are constantly I 



being discovered, any such would be but premature, and 

 we must be content with that laid down for us by 

 Kolliker. Had the value of the labours of Valenciennes 

 been properly appreciated, this might not now be the 

 case. The almost overcrowded plates have been drawn 

 by H. Bucher, Jun., with all that skill which we have before 

 admired, though perhaps the drawings of the spicules 

 convey too much the notion of their being perfectly solid. 

 We shall wait with great expectancy the publication of 

 future memoirs of the other families of the Alcyonaria. 



THE COLOURS OF THIN PLATES} 



'X'HE physical theory, as founded by Young and per- 

 ^ fected by his successors, shows how to ascertain 

 the composition of the light reflected from a plate of 

 given material and thickness when the incident light is 

 white ; but it does not and cannot tell us, except very 

 roughly, what the colour of the light of such composition 

 will be. For this purpose we must call to our aid the 

 theory of compound colours, and such investigfttions as 

 were made by Maxwell upon the chromatic relations of 

 the spectrum colours themselves. Maxwell found that on 

 Newton's chromatic diagram the curve representative of 

 the spectrum takes approximately the simple form of two 

 sides of a triangle, of which the angular points represent 

 a definite red, a definite green, and a definite violet. The 

 statement implies that yellow is a compound colour, a 

 mixture of red and green. 



In illustration of this fact, an experiment was shown 

 in which a compound yellow was produced by absorbing- 

 agents. An infusion of litmus absorbs the yellow and 

 orange rays ; a thin layer of bichromate of potash re- 

 moves the blue. Under the joint operation of these 

 colouring-matters the spectrum is reduced to its red and 

 green elements, as may be proved by prismatic analysis ; 

 but, if the proportions are suitably chosen, the colour of 

 the mixed light is yellow or orange. When the slit of the 

 usual arrangement is replaced by a moderately large cir- 

 cular aperture, the prism throws upon the screen two 

 circles of red and green light, which partially overlap. 

 Where the lights are separated, the red and green 

 appear ; where they are combined, the resultant colour 

 is yellow. 



On the basis of Maxwell's data it is possible to calcu- 

 late the colours of thin plates and to exhibit the results 

 in the form of a curve upon Newton's diagram. The 

 curve starts at a definite point, corresponding to an in- 

 finitely small thickness of the plate. This point is some- 

 what upon the blue side of white. As the thickness 

 increases, the curve passes very close to white, a little 

 upon the green side. It then approaches the side of the 

 triangle, indicating a full orange ; and so on. In this 

 way the colours of the various orders of Newton's scale 

 are exhibited and explained. The principal discrepancy 

 between the curve and the descriptions of previous ob- 

 servers relates to the precedence of the reds of the first 

 and second orders. The latter has usually been con- 

 sidered to be the superior, while the diagram supports 

 the claim of the former. The explanation is to be found 

 in the inferior brightness (as distinguished from purity) 

 of the red of the first order, and its consequent greater 

 liability to suffer by contamination with white light. 

 Such white light, foreign to the true phenomenon, is 

 always present when the thin plate is a plate of air 

 inclosed between glass lenses. To make the comparison 

 fairly, a soap film must be used, or recourse may be had 

 to the almost identical series of colours presented by 

 moderately thin plates of doubly-refracting crystals when 

 traversed by polarized light. Under these circumstances 

 the red of the first order is seen to be equal or superior 

 to that of the second order. 



' Abstract of Lecture delivered by Lord Rayleigh at the Royal Institution 

 on March 25, iBf;. 



