March. 1911. 



KNOWLEDGE. 



113 



will see amongst the British Birds, specimens of Coal-Tits 

 with freshly-written labels bearing the name Irish Coal-Tit 

 [Pants liihcrnicns). Mr. W. R. Ogilvie-Grant has recently 

 described this bird as a new species and thns named it 

 (Bulletin British Ornithologists' Clith, Vol. XXVII, pages 

 36 and 37), on the strength of certain differences from the 

 British Coal-Tit. It wonld seem, however, that the new form 

 has yet to make good its claim to specific rank, sound critics 

 being inclined to regard it rather as a geographical form. 



This makes the third Irish bird which has recently been 

 described and named as distinct from recognised species. The 

 other two are the Irish Dipper named Cinclns ciiicliis 

 hibernicHs. by Dr. Ernest Hartert, and the Irish Jay named 

 Garriiliis glaiulariiis hihcniiciis. hv the same writer and 

 Mr. H. F. Witherby. 



HOW MAW KINDS OF BRITISH BIRDS ARE 



THERE? — The latest anthorhative "List of British Birds" 



is by Mr. W. R. Ogilvie-Grant (1910) and ennmerates four 



hundred and forty-two species. An analysis of the grouping 



adopted by the writer (which is the same as that used in 



the Natural History Museum, South Kensington), gives the 



lollowing instructive figures : — 



133 species are resident and breed. ) in-u j- 

 - -, • , . . =183 breedmg 



32 species are regular summer visi- I- . " 



tors, and breed I 



55 species are regular autumn, winter or spring 



\isitors ; not breeding. 



9 species are occasional visitors ; used to breed. 



192 species are occasional visitors; never known to 



breed. To this number the American Pipit, 



mentioned in the last number of " Knowledge " 



(page 76) falls to be added. 



1 species is e.xtinct (Cireat Ank). 



Total 442 species admitted nnciucstionably. 



3 varieties are named in the List, but not numbered. 

 35 species, of which thehistorj'is doubtful, are named 



in the List within brackets, and are not numbered. 



4S0 birds in all are thus found named in this List. 



In the scientific names throughout this List, binomials 

 only are used, the author having, with sound judgment, not 

 adopted the new trinomials which are being used now by 

 various writers. 



NOTES FOR CORRESPOXDKXTS.— D.N. H.,Chesham. 

 — Owls are not uncommon at Hampstead, and recently I 

 have heard two kinds calling. On 1st May, 1910, I saw a 

 "Brown" Owl (probably Syrniu}!! aliico), seated high up in 

 an ash tree in Piatt's Lane, mobbed by two noisy Mistle- 

 thrushes and a Blackbird, which it treated with unmoved 

 contempt. The Barn-Owl (Strix fianuiica) is known to nest 

 each season, and the Long-eared Owl {Asis otns) has nested 

 in Ken Wood. 



The M.ARSH-TiT [Pants paliistris. or, if a trinomial is 

 used, P. p. drcsscri) is not rare in Bucks, and nests, but it 

 may be local in its distribution, as it is in other districts. If 

 the bird you saw was feeding it may have been taking small 

 molluscs, as it is almost omnivorous. 



Two forms of " Marsh " Tit are now recognised in this 

 country. The new one is named the British Willow-Tit 

 [Parus klcinschiiiiilti or P. atricapillns klcinschniidti'i. 

 and has been recorded from so many places that a careful 

 examination of all " Marsh " Tits met with should be made by 

 observers. For description, see Witherby's " British Birds," 

 Vol, I., pages 23, 44, and 214 (1907), and'Vol. IV., pages 146 

 and 284 (1910-11). 



PHOTOGRAPHY. 



By C. E. Kenneth Mees. D.Sc. 



STANDARD LIGHT SOURCES.— Inasmuch as photo- 

 graphic investigations are mainly concerned with the 

 measurement of the effects of light, it is naturally of great 

 importance that a standard source of light of known strength 

 and constancy should be readily available. Unfortunately, 



neither the production nor the measurement of light are 

 susceptible of the very accurate control possible to other 

 sources of energy, the accuracy of measurement being limited 

 by the difl'erence in intensity which can just be perceived bv 

 the eye when two adjacent patches are illuminated ; while the 

 production of a standard light depends at present on a set 

 of definitions of a standard lamp, which has resulted in the 

 introduction of a large number of different standards more 

 or less harmonized by photometric comparisons. 



There is, indeed, no international standard of light in the 

 sense in which the " volt " and " ampere " form electrical 

 standards : although the British standard which is know-n as 

 the '" candle," and in practice is the light of a Pentane lamp, 

 which has been adopted by most countries, the chief exceptions 

 being Germany and Austria, of which the standard is the 

 " hefner," the light of an Amyl-acetate lamp, of which the 

 intensity is nine-tenths that of the standard candle. 



Attempts have been made to introduce as international 

 standards the light given by platinum at its melting-point, 

 or by platinum foil heated by a standard current. But the 

 practical difficulties attending the use of these methods seem 

 to have prevented their general employment. 



F"or photographic purposes, what is required is an accurate 

 secondary standard, but the difficulties here are even greater 

 than in visual work because lamps of different kinds and of the 

 same visual intensity, do not produce at all the same effect 

 upon a photographic plate, owing to differences in the spectral 

 quality of the light produced. 



The light by which photographic plates are generally used 

 is da\light, and this differs so greatly in composition from 

 artificial light sources that comparisons of plates made by 

 artificial light sources maybe most misleading when the plates 

 are used bv daylight, especially in the case of orthochromatic 

 or panchromatic plates. This difficulty may be overcome by 

 special methods of screening the light, and wdiat is required 

 for photographic investigation is, therefore, a method of 

 producing light sources of constant intensity, which can be 

 referred to some priinary standard. The intensity of these 

 light sources should be between one and four candles and 

 they should be capable of burning for at least fifteen minutes 

 without attention and without a variation greater than one 

 per cent. 



When the subject was discussed at the last International 

 Congress of Photography the general opinion of the speakers 

 seemed to be that the choice for such a secondary standard 

 lay between acetylene burners and small metal filament 

 electric lamps : the former being largely used in France and 

 England, while in Germany the electrical standard is more 

 favoured. An acetylene burner giving a single cylindrical jet 

 of flame, from which a small portion is selected by means of a 

 hood having a horizontal slit, will easily give an accuracy of 

 one per cent, and has many advantages ; notably, that it will 

 burn for a long time without any attention, pro\ ided that the 

 pressure in the generator is constant. The chief disadvantage 

 of acetylene standards is that for accuracy they require very 

 large generators and they are consequently rather expensive to 

 instal, while it is impossible to render them portable. 



-An examination of small four-volt metallic filament lamps 

 has shown that they will be constant to one per cent, if the 

 voltage is maintained constant to a one-hundredth of a volt 

 which can be done with comparative ease by means of a 

 potentiometer, and as different lamps can be arranged with 

 scries resistances to give the same candle pow-er for the same 

 impressed electro-motive force, it should be possible to 

 prepare an electric secondary standard without great difficult)'. 



The chief objections to these small lamps as standards — the 

 rapidity with which they age — is of little importance in photo- 

 graphic work, where the total duration of exposures in a day's 

 work wonld be exceedingly small. 



It may be remarked that there seems to be an increasing 

 tendency to use small metallic filament lamps, run oft" accumu- 

 lators, as standards, especially in simple photometers of the 

 portable tj'pe, and that in many cases these lamps are simply 

 assumed to be constant without any control whatever. They 

 are, in fact, very far from constant, not only gi\ing a different 

 intensity after the accunn:lator is charged, or when it is nearly 



