\\6 



KNOW I.I. i)r,i:. 



Makcii, 1912. 



.\ c"()Nvi:vi:i) n:Ki:(".KiNi:. — Oil ihc Mioini.ij; of 



29lli Dccfiiihcr lasl a I'crtriiiic was foiiiul llyint,' ioiiikI diic of 

 the looms ill the General I'ost Olhcc, ClasKow. hnl this 

 pu/ziiiiK occurrence has been nicely explained hy Mr. John 

 Paterson. He writes: — "When the S.S. Anchorin of the 

 Anchor Line, on her maiden trip to Calcutta, had been in the 

 Red Sea a couple of days, a hawk was seen on the top of one 

 of the masts Keltinii; an assisted passajje on its southward 

 journey in the wake of the migrating flocks from the north, on 

 whom it naturally preys." The bird was secured by a sailor; 

 and in a bo.v, fitted as a cage, " made the journey to Calcutta 

 and (ilasgow, by way of some Italian ports." It was pre- 

 sented bv the captain of the steamer to Mr. Paterson, in 

 Glasgow, who was curious to exactly know what kind it was. 

 He found it to be a ''young peregrine falcon .... or 

 passage-hawk, the name falconers gave to a young bird caught 

 during the season of migration." Having satisfied liitiis-lf. 

 Mr. Paterson thought to give the bird 

 freedom and accordingly let it ofl 

 within the city of Glasgow. Such 

 freedom as the ;itmosphere of Glas- 

 gow may afford to a falcon was 

 apparently not congenial, and the 

 following morning found the bird in 

 the toils again, as above mentioned. 

 The case is a good illustration of 

 how " casual " introductions may be 

 made into our fauna, by man's agency. 



WADING HIRDS INLAXO ON 

 MIGRATION.— In Kast Renfrew- 

 shire a group of small upland 

 reservoirs, of which Balgray Dam is 

 the largest, has been long known to 

 Clyde bird-men as a resort of waders 

 and other water-birds. Mr. John 

 Robertson, who has paid particnln 

 attention to the locality, reports tli.n 

 the autumn of 1911 was the be.i 

 season there for waders in his ox 

 perience of sixteen years. Tin- 

 shortage in summer rainfall led to 

 the exposure of so great an area of 

 the bottom of Balgray Dam that 

 passing birds were attracted to it 

 as a feeding ground in greater 

 numbers than usual. Mr. Robertson 

 considers these birds to be making 

 their way from the Forth to the 

 Clyde ; that is to say that these 

 dams are a point on a line of 

 migration-flight across Scotland. The 

 spot is not particularly favoured in natural situation and 

 surroundings, being on the edge of the extensive industrial 

 area of the lower Clyde valley, and almost in touch with 

 suburban Glasgow, but no fewer than twenty-two species 

 of waders have been noted there. Of these Mr. Robertson 

 observed seventeen last autumn, from August to October. 

 The movement began with the appearance of Ringed and 

 Golden Plovers on 30th July, and the most noteworthy birds 

 seen were the Turnstone, Curlew-Sandpiper (up to 20 in 

 numbersl. Knot (up to 24), Ruft", Green Sandpiper. Greensliank. 

 and Bar-tailed and Black-tailed Godvvits. In previous years 

 the Grey Plover, Little Stint and Spotted Redshank had been 

 seen. (John Robertson — The Glasgow Xaturalist, Novem- 

 ber, 1911. volume IV, pages 7-10. Glasgow: John Smith & 

 Son, Ltd.) Of the above-named, it may be remarked that it is 

 quite exceptional to find the Turnstone, Knot, and the two 

 Godwits inland in Scotland. 



PHOTOCik.Al'llV. 



By Edgar Senior. 



PHOTtJGR.VPIlING THE INVISIBLE.— Mysterious as 

 the above title may appear, it has no connection with ghosts 

 or spirit photography, but is merely intended to infer that, as 



\ Portrait takni witl 



far as the eye could discern, no light whatever reached the 

 photographic plate. Such, however, were the conditions 

 present, that plenty of rays which our eyes had no cogniz- 

 ance of, were able to do so and act photographically upon it. 

 As far back as 1801, Ritter, of Jeiia, proved the existence of 

 rays having very powerful photographic properties occupying 

 a region beyond the extreme violet end of the spectrum, which 

 do not excite the organs of vision, and to which the iiaiiic 

 ultra-violet is given. To Sir George Stokes we owe the dis- 

 covery of how to make this region apparent. The beautiful 

 experiment consists in placing a card, which has been coated 

 with (luinine sulphate made slightly acid, in their path, when 

 rays which were previously invisible at once shine forth, and 

 the extent of the spectrum beyond the violet becomes apparent. 

 That these ultra-violet rays have powerful chemically active 

 properties is well known, their behaviour towards silver 

 chloride bring of special interest. If a piece of fused silver 

 chloride be placed in front of the slit 

 of a spectroscope, the light that passes 

 through forms a spectrum apparently 

 unaltered in range from the ordinary 

 one. but if a card coated with a solu- 

 tion of (juinine sulphate be placed in 

 position beyond the violet no effect is 

 produced, the invisible rays appear 

 entirely absent ; but the removal of 

 the slab of silver chloride at once 

 makes them appear, and from this we 

 gather t hat the silver salt has absorbed 

 them, and that chemical action will 

 consequently take place in this region. 

 In fact, the violet and ultra-violet rays 

 .'lie the most active upon silver salts 

 generally, as well as those of iron, 

 uranium, chromium, and so on, and 

 to such an extent is this the case, 

 that formerly the violet end of the 

 spectrum was regarded as the 

 seat of chemical energy, and the 

 name actinic rays applied to that 

 region. This, however, can no 

 longer be the case, as all parts 

 of the spectrum are actinic, accord- 

 ing to the nature of the sensitive 

 compound employed. The idea of 

 taking advantage of the sensitive 

 iiess of silver salts to these rays 

 of short wavelength, and so obtaining 

 photographs of things invisible to 

 our eyes is almost as old as plioto- 

 graphy itself, for we find the 

 suggestion of the experiment outlined 

 in the " Pencil of Nature," published by Fox Talbot in KS44. 

 .•\fter describing the production of a solar spectrum and 

 the effect produced upon a sheet of sensitive paper by the 

 rays beyond the violet "whose existence is only made known 

 by the chemical action they exert," he suggests the possibility 

 of separating them from the rest by passing them through an 

 aperture into an apartment, or room, and so filling it with 

 invisible rays, when, if a camera were so placed as to point in 

 the direction of any objects, photographs might be taken by 

 moans of the action of the in\isible radiations upon the sensi- 

 tive plate. Talbot thus believed that it would be possible for 

 a person seated in a totally dark room to have a portrait 

 taken in the ordinary manner, or, in other words, that we 

 should be able to photograph the invisible. The very 

 fascination of the subject induced the writer. " some years ago 

 now," to experiment in the direction indic;itcd. with the result 

 that among those photographs produced in this way that of 

 greatest interest " from its close connection with the ideas of 

 Talbot," is the one that is reproduced abo\e to form Figure 12i. 

 Although not carried out in quite the same manner as that 

 suggested, the principle remains the same, as only those 

 dark rays which exist beyond the violet were utilized, .ill 

 visible light being cut otT by means of a screen devised by 

 Professor R. \V. Wood combining a piece of cobalt blue glass 



\"iolet Ravs. 



