6i6 



KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1906. 



ridibundus), with a view to ascertaining if it is harmful to 

 the fishing or farming industries, .'\ccordingly, these 

 gentlemen have issued a' circular requesting answers to the 

 following questions : (i) Do you consider the black-headed 

 gull harmful to fishing or farming industries? State 

 reasons. (2) Have vou ever examined the gullet and 

 stomach of this gull? If so, what were the contents? 

 {3") What, in your opinion, is the staple food of this gull ? 



Under the circumstances, this method of obtaining in- 

 formation is about the only possible one, but it is, neverthe- 

 less, most unsatisfactory and unreliable. Of the few people 

 who have ever examined the contents of the stomach of 

 this gull, for example, how many took note thereof, and 

 of these, how many were really qualified to analyse these 

 contents accurately? And on this question the whole case 

 for protection, or otherwise, rests. 



This dallying with the question of protective or repres- 

 sive measu'resin this matter is to be greatly deplored. It 

 is a question for the Board of .Agriculture, who should have 

 appointed, long since, an expert, or experts, to study, im- 

 partially and exhaustively, the whole matter of the relation 

 of our native birds to the industries. For years past this 

 line of investigation has been prosecuted by a specially 

 appointed Bureau in the United States, and the results they 

 have achieved are of the highest importance and value. 



It must not be forgotten that under the conditions exist- 

 ing to-day in this country any given species of bird may 

 well exceed its proper limits ; when exhausting its normal 

 food supply, it may be obliged to tap others, and so become 

 a nuisance. But it is not necessary that new supplies 

 should be tapped ; it may become no less a nuisance by 

 drawing too largely on its legitimate food, where this brings 

 it into competition with man, as in the case of fish-eating 

 birds, for example. .Vnd in such cases it must certainly be 

 kept in check. 



But the " opinion " of the majoritv of the so-called ob- 

 servers of birds is absolutely worthless. 



Greater Yellow Shank in Scilly. 



.■\t the first meeting of the winter session of the British 

 Ornithologists' Club, an adult specimen of the greater yel- 

 low shank {Totaiuis vicla)whiuys) was exhibited, which 

 had been killed at Tresco, Scilly Islands, on September 16, 

 by Captain Doreen Smith. This makes the first record of 

 this species either in Great Britain or the Continent. The 

 sex of the bird does not appear to have been determined. 



Waxwing in Wales. 



In the Field (November 17), it is recorded that a wax- 

 wing was seen on November 7, near Bala, in North Wales. 



Hoopoe at Whitby. 



On November 13, Messrs. Rowland, Ward and Co., of 

 Piccadillv, received for preservation a male hoopoe, which 



had l.iccn shot on the moors near Whitby. 



Little Owl in Hertford and Elsewhere. 



Messrs. Rowland, Ward have also received a little owl, 

 a female, shot at Bramfield, near Hertford, on October 26, 

 and another, a male, shot near Newport, Essex. Another 

 is recorded (Field, November 17) to have been seen in 

 Derbyshire on November 5. Since imported examples of 

 this species have been repeatedly set at liberty in this coun- 

 try, it becomes doubtful whether the birds here referred to 

 are genuine wild birds or the progeny of the liberated cap- 

 tives. 



PHYSICAL. 



By Alfred W. Porter, B.Sc. 



Haidinger*s Interference Rings. 



In the Philosophical Marjaziiie for November, Lord Ravleigh 

 points out that Haidinger was fully acquainted with the 

 character of his rings and especially the distinction between 

 them and the rings named after Newton and dependent 

 upon a variable thickness in the thin plate. The latter are of 



the same kind as the colours on a soap film, the variety of 

 colour being due to a corresponding variety in the thick- 

 ness of the film ; the colours appear as though painted on 

 the film. Ilaidinger's, on the other hand, are obtained 

 with a perfectly parallel faced film, such as a thin sheet 

 of mica or selenite, and are due lo the different obliquities 

 of the bundles of interfering rays. Contrary to the case of 

 Newton's fringes, they are localised at infinity. The usual 

 text-book formula for the retardation between the interfering 

 lays is : 



Retardation = 2 yn e cos. r, 

 where/ii is the refractive index of the film, c is its thickness, 

 and r the angle the ray in the film makes with the normal. 

 The retardation, and, therefore, the particular colour effect, 

 varies with e, and this represents Newton's case. It also 

 varies with r when c is constant, and this is Haidinger's 

 case. Of course, all manners of intermediate effects are 

 obtained when e and r both vary. Lord Rayleigh gives the 

 following directions for best seeing Haidinger's rings by 

 transmission : — " The transmitted rings are best seen by 

 holding the mica close to the eye (focussed for infinity), and 

 immediately in front of a piece of finely-ground glass 

 behind which is placed a salted Bunsen flame." Plates 

 about a fifth of a millimetre thick are suitable. We may 

 point out that a very effective way of seeing them is to 

 replace the ground glass by a lens (a pocket magnifier 

 serves very well), held in front in such a position as to 

 appear " full of light"; the mica, as before, being close to 

 the eye. Those in possession of a mineralogical microscope 

 can obtain the rings very perfectly by placing the mica on 

 the stage and e.xamining the back focus of the objective by 

 means of the additional lens usually supplied for the pur- 

 pose, which is inserted in the tube; the illumination, as 

 before, must be monochromatic. Lord Rayleigh describes 

 some novel effects. Mica is bi-refringent, and, in conse- 

 quence, there are two sets of rings. These may nearly 

 annul one another for certain thicknesses of mica. When 

 this is the case, by employing a Nicol prism, which can be 

 rotated, there are four positions for which the inner rings 

 become distinct. 



The rings formed on the same mode as Haidinger's are 

 the ones which are employed in Michelson's, and in Fabn,' 

 and Perot's Interferometers, for the absolute determination 

 of the wave-lengths of light. 



Haidinger's Tufts. 



The name of Haidinger has also been prominently 

 brought forward in another connection. In the Proceedinqs 

 of the Royal Dublin Society, Professor W. F. Barrett dis- 

 cusses the tufts or brushes which are known bv the above 

 name. When a bright sky or any brightly illuminated sur- 

 face is looked at through a Nicol prism, a pair of small, 

 yellow cones, joined apex to ape.x, are seen in the direct line 

 of vision. At right angles, and filling the larger space on 

 each side of these cones, a faint blue or violet colour is 

 seen. The yellow tufts resemble an hour-glass. Their 

 longer axis rotates if the Nicol is rotated. If the Nicol is 

 kept unmoved before the eye, the effect vanishes in a few 

 seconds; if the Nicol be now suddenly turned it reappears 

 and again fades. This is the origin of the difilculty which 

 some people experience in seeing the tufts. To ensure their 

 appearance. Professor Barrett recommends that the Nicol 

 be turned backward and forward every few seconds, and a 

 piece of cobalt blue glass be interposed. 



When they have been found, they may still be seen even 

 though the Nicol be removed. This is, no doubt, due to the 

 partial polarisation of the light reflected from the white 

 backgiound. These brushes are closely associated with the 

 yellow spot (or fascicula lutea), of the eye. That their de- 

 tection is due to the retina is demonstrated by the following 

 experiment : An opaque screen with two pin-hole aper- 

 tures, each I mm. in diameter and 2 mm. apart, held be- 

 tween the eye and the Nicol prism, would give a double 

 image of Haidinger's tufts if they were due alone to the 

 refracting media of the eye ; but only a single image is 

 seen ; hence their detection is due to the retina. The size of 

 the region of the retina which is effective can be found 

 by measuring the length of their projected image on a 

 sheet of white paper, placed at a known distance from the 



