September 1, 1897.] 



KNOWLEDGE. 



221 



In conclusion I may sum up briefly the principal points 

 in this paper, which has run to much greater length than 

 was at first intended. 



i. The absolute intensity of the image of a celestial 

 object, and therefore the absolute photographic light action 

 (product of intensity x time) for a given time of exposure, 

 will vary: (1) lor extended sources, as the square of the 

 anovular aperture only ; (2) for point sources, as the pro- 

 duct of the square of the angular aperture into the square 

 of the linear aperture. 



II. The contrast between the image of any celestial 

 object (not very near the horizon) and the general field 

 depends upon il) the brightness of the sky at the time ; 

 (2) the efficiency of the image-forming lens as regards 

 perfection of figure, nature, and curvature of surfaces, etc. ; 

 (3 1 and upon the square of the linear aperture. If the 

 objectives are good, the sky effect (1) and (2) is the most 

 important. 



in. For extended objects, such as nebulas, irresolvable 

 star clouds, etc., etc., in which we have to deal with the 

 delmeation of a surface rather than with individual points, 

 this cotHrast can only be increased by decreasing the focal 

 length. When the sky effect (1) and (3) is predominant, 

 it will vary inversely as the square of the latter quantity. 



IV. For point sources the contrast can only be increased 

 by increasing the angular aperture. Under the same con- 

 ditions as in II., it will vary directly as the square of this 

 quantity. 



V. It is the der/ree of contrast and not the size of objective 

 (except in so far as this latter influences the contrast) that 

 determines the limiting magnitude of the faintest object 

 that can be photographed. This limiting magnitude for 

 stars depends, therefore, on the angular aperture of the 

 instrument ; and for nebulfe only on the focal length. 



VI. The time of exposure also depends very largely on 

 the contrast between image and field, and not on the 

 absolute intensity of the former. If the development 

 could be suited in every case to the exposure, the time of 

 the latter necessary to obtain any given degree of detail 

 would vary inversely as the contrast within a wide range of 

 photographic light action. 



VII. As regards the influence of the character of the 

 objective (II. (2) ) upon the illumination of the field, the 

 refractor seems to have a decided advantage. The angular 

 aperture of the latter should not, however, be greater 

 than one-fifth. To photograph the very faintest stars 

 (beyond the seventeenth magnitude) a reflector of the 

 largest possible angular aperture — one-third or one-half if 

 possible — is the onl'i instrument that can be used. 



CoNXLrsiox. — In this paper I have carefully abstained 

 from any mathematical exposition of the various relations 

 between aperture, focal length, and intensity of image and 

 field. Those who prefer such a method of treatment will 

 find it, together with some further developments of the 

 present paper, in an article " On the Conditions of Maxi- 

 mum Efiiciency in Astrophotographic Work," in the 

 August Number of the Astrophysical Journal. 



THE BIRDS OF OBAN'S ISLES. 



By Habey F. Witherbt. 



LYING in the Firth of Lome, and within a radius of 

 ten miles or so of Oban, there are a number of 

 tiny islands which are well deserving of a visit 

 from the ornithologist. 



It was on May 25th that I started out 

 from Oban Bay with two friends, in a ten-ton cutter, to 



explore these islands ; and I intend here to give a short 

 account of what we did, and of what we found, during the 

 ten days in which we were cruising among the islets. 



The first day we ran down to "Black Isle," so called 

 because of its dark appearance at a distance. We stayed 

 here all day and thoroughly explored the island. 



For the benefit of those who have not visited these or 

 similar islets, I may here describe then* general character. 

 There is usually no beach at all, seaweed-covered rocks 

 dipping sharply into the sea surrounding the island. 

 Above the seaweed there lies a belt of sloping rock covered 

 with lichens of various shades of grey and yellow. Beyond 

 this sloping fringe of rock one comes to more or less flat 

 ground covered with grass, heather, and other under- 

 growth. Occasionally there are a few stunted trees on the 

 island, and perhaps a small hill. Sometimes, too, the 

 island itself rises on one side in a sheer cliff of eighty 

 feet or more. 



Xest of the Common Gull. 



On Black Isle — as, indeed, on most of the islands roimd 

 Oban — we found the fringe of lichen-covered rock tenanted 

 by gulls (common, lesser black-backed, and herring) and 

 oyster-catchers. The gulls' nests were very often ex- 

 ceedingly prettily placed. Built of withered grass, moss, 

 and lichen, and containing two or three greenish brown 

 spotted eggs, they were very conspicuous ; yet, in colouring 

 they were in perfect harmony with the light grey rock, 

 covered with bright lichens and brilliant sea-pinks. 



The oyster-catcher — or, more appropriately, the mussel- 

 picker — generally lays its eggs in a little hollow scooped 

 among the pebbles on a beach, but it also nests in con- 

 siderable numbers on small islands off the mainland. 

 Even should there be a beach on the island, it is my 

 experience that the oyster-catcher does not use it as a 

 nesting place, but invariably lays its eggs either in a small 

 hollow in the bare rock, or in a scoop in the short thick 

 turf. When using such places as a nesting site the oyster- 

 catcher places at the bottom of the scoop a few pebbles or 

 broken shells, or, as I have often seen, some dry rabbit 

 dung, which it no doubt mistakes for pebbles. 



This habit of placing stones at the bottom of the nest 

 seems to indicate that in nesting on rock or grass the 

 oyster-catcher is merely adapting itself to circumstances, 

 and that the pebbly beach was its original breeding place. 



In one of the oyster-catcher's nests which we found, the 

 eggs were just hatching, and as I took one up in my hand 

 and put it to my ear, I could distinctly hear the young one 

 inside the shell shrilly uttering exactly the same note 

 which the old bird was loudly calling overhead. In hia 

 book on " The Evolution of Bird Song," Mr. Witchell 

 mentions a similar instance of a lapwing in the shell 



