October, 1906.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



569 



Flags, were collected at Cochaiya, about 3 miles north-east 

 of Pata. New species of Symphysurus and Trinucleus, 

 probably of Arenig age, were found about a mile from 

 Apolo, Province of Caupolican. An indeterminable species 

 of Oijygia was obtained from the right bank of the River 

 Caca, in the same province. Fhacops cf. arhufeus, Dal- 

 manifus I'aituna, and V. Maecurna were collected in the 

 track from Apolo to San Jose de Chupiamonas, also in the 

 province of Caupolican. The nodules from which they 

 were derived are probably of Lower Devonian age. It is 

 worthy of remark that, while the earlier forms show 

 aflinities with the contemporaneous European fauna, the 

 Devonian species are much more closely allied to those of 

 South Africa and North America. 



Vegetable Structures in Coral Reef. 



Considcrabli- interest is being shown in the light thrown 

 upon the origin of limestone in the researches conducted by 

 Messrs. Frederick Chapman and Doug'as .Mawson, in re- 

 gard to the importance of HaJimcda as a reef-forming 

 organism. The freshwater Chara, calcareous alga;, 

 nullipores, and purple sea-weeds of the type of Liiho- 

 tliamnion have long been known to form the basis of lime- 

 stones and coral-reefs, but in Halimeda we have a cal- 

 careous green alga performing a similar operation. The 

 material obtained in the great boring, the lagoon-borings, 

 and lagoon-dredging at [""unafuti has yielded a consider- 

 able quantity of Halimeda ; and Dr. Guppy has described 

 a Halimeda-limeftone in the Solomon Islands. Evidence 

 such as this shows that the important deposits of calcareous 

 plant-remains forming at the present day can scarcely be 

 paralleled by any deposit formed in past geological times, 

 except, possibly, the limestones of the .-Mpine Trias, which 

 owe their origin to the thallophytes Diplopora and 

 GyroporeUa. .^mong■ other ffaKmcda-limestones may be 

 mentioned those of Christmas Island, Fiji, and Tonga, and 

 the New Hebrides, but these differ considerably one from 

 the other in the condition of preservation of their chief 

 organic contents. Halimeda seems to be more liable to 

 decay than Liihothamnion, corals, or foraminifera, and \et 

 it appears to retam its structure to a considerable depth in 

 reefs. Much of the fine powdery limestone associated with 

 coral-reefs, and more especially with upraised coral-islands, 

 may be primarily due to lagoon and other deposits formed 

 by the agency of Halimeda. 



ORNITHOLOGICAL. 



By W. P. Pycraft, A.L.S., F.Z.S., M.B.O.U., &c 



Herons and Fish Preservation. 



Some little lime since, the Dart District 1-ishery Board 

 issued urgent a|jp(als to the owners of heronries to reduce 

 the number of their herons on account of the damage sup- 

 posed to be done by their ravages. 



The members of the Board do not seem to have been 

 very well informed on the matter, as is usuallv the ca.se. 

 For the replies to their requests showed that at least one 

 heronry existed only in the imagination of the Board, while 

 another boasted but half-a-dozen birds, which the owner 

 rightly refused to interfere with. .As a matter of fact, the 

 evidence all tended to show that the heron was by no 

 nil ans a common bird on the Dart. 



We cannot but regret the fact that sport should so com- 

 monly go hand in hand with greediness. This stately 

 bird is one of the few remaining relics of our native fauna 

 which has been slowly strangled to ensure larger bags to 

 tile " sportsman." Ilajipily, however, there areniany who, 

 while keen sportmen, are no less keen naturalists, and to 

 them we owe what is left to us of our larger native birds 

 and beasts. 



Red Crested Pochard in Norfolk. 



Mr. .\'. Ilerb<Tt Sinitli, in the Field, September 15, re- 

 cords the fact lh;it a pair of red-cresled pochards {Netfa 

 rufina) were killed by his keeper at HickliEig, on September 

 S. The first record of this bird as a British species dates 

 back to 1818, when .1 female was killed on Breydon Water, 

 in Norfolk, in the month of Julv. The present example 

 m.-ide the tenth record for this ci'uinty. 



Nine other ex;itnpU's however, all males, have now to be 



added to this list. The Rev. .M. C. H. Bird, recording the 

 fact in the Field, September 22, that nine adults, six males 

 and three females, of this species were killed on September 

 4, out of a flock of thirteen, on Breydon Water, Great Yar- 

 mouth. " Supposing," Mr. Bird writes, " as is probable, 

 that the pair killed at Hickling on September 8 . . . 

 were part of the above company, only one of the ' bakers' 

 dozen ' of these rare visitants remains unaccounted for." 

 To this the editor of the Field pertinently remarks : " What 

 a pity it is they should have to be ' accounted for,' instead 

 of being left alone to delight hundreds of other people 

 besides the shooter! " 



Hoopoe in Cheshire. 



Mr. .\lfred Newstead records [Fi'id, September 15) the 

 fact that a hoopoe was captured near Chester on August 

 23, and remarks that this is only the third known instance 

 of its occurrence in Cheshire. 



Alpine Swift in Devon. 



Two Alpine swifts (Cypsilns nlpinux) are reported (Field, 

 September 22) to have been seen about a mile from 

 E.xmouth during ;\ugust. The large size of these birds, and 

 their very distinctive coloration, makes it unlikely that any 

 mistake has occurred in t!ie correctness of this record. 



rrrrr* 

 PHYSICAL. 



By Alfred W. Porter, B.Sc. 



Mechanical Analogue of a Diffraction 

 Grating. 



In the Astropln/Kieal JuKnud for July i\Ir. H. M. Reese 

 describes an experiment in illustration of the performance of 

 diffraction gratings. " The schenu- was to produce, on the 

 surface of mercury, ripples emanating from a series of 

 equidistant points distributed along a straight line." To 

 produce these a thin sheet of iron was cut into the form of 

 a comb of sixteen teeth, spaced 5 mm. apart, and attached 

 to the lower prong of an electrically-driven tuning-fork 

 arranged to vibrate in a vertical plane. This comb was set 

 near the edge of a tray of mercury in such a way that the 

 teeth would dip into the surface. When the ripples were 

 examined stroboscopically it was seen that near the comb 

 they mixed together chaotically ; but at a moderate distance 

 they resolved themselves into several series of regular recti- 

 linear wave-trains, which advance in different directions 

 and are symmetrically distributed. These correspond to 

 the spectrum of zero order — moving out in a direction 

 normal to the comb — and spectra of orders one and two on 

 each side. .Vn image of the ripples can be projected on a 

 screen. To do this most successfully it must be arranged 

 so that the projecting lens receives as much light as possible 

 from the source. This is brought about if a converging 

 beam from a lantern is incident upon the mercury and the 

 projecting lens is placed at the point of convergence of this 

 br.im aftiM- relleclion. 



Distribution of Radiation from Radioactive 

 Sources, 



The sun appears as bright near the middle as near the 

 edge; so does a round hot poker. This is a consequence of 

 the cosine law, which states that the radiation from any 

 very small clement of the luminous body varies according 

 to the cosine of the angle between the norm.il to the surface 

 and the direction in which the element is viewed. The 

 reason the element does not radiate equally in all directions 

 is that the radiation comes not from the surface onl\-, but 

 from a perceptible depth ; and in oblique directions the r.adia- 

 tion sulTers most loss from absorption by the layer of sub- 

 stance through which it passes. If it came only from the sur- 

 face there is no reason why it should not spread out equally 

 in all directions, and in ih.at case the sun would not appear of 

 uniform brightness. Prof. Rutherford has recently {Philo- 

 sophical Mayaziitr, .August, 1906), explained in this way 

 the peculiar appearance of the image formed by the alpha 

 stream from radioactive bodies. .V wire is made radioactive 

 by exposure to radium emanation. The alpha radiation 

 from it is allowed to pass in vacuo through .a verv narrow 

 slit to a photographic plate, where it makes what is essenti- 

 ally a " pin-hole " photograph. Now the action on the 



