356 



KNOWLEDGE. 



September, 1911. 



Cniversity of Aberdeen. Properly controlled methods and 

 results should accordingly be assured, and, while Mr. Thomson 

 writes as an enthusiast, he does so judiciously, and does not 

 overlook the difficulties nor exaggerate the possibilities of 

 the subject. 



For some of the .Aberdeen returns already reported see 

 " KNOWLEDGE" for May. 1911 (page 198). 



We wonder wh\- so few or no returns seem to be forthcoming 

 from one class of birds, viz.. the game-birds which, owing to 

 the numbers meeting death from the hands of man, would 

 seem to be liliely to yield a larger percentage of returns to 

 the numbers marked than any other class of birds. We 

 do not overlook the returns regarding Woodcock, but refer 

 more particularly to Grouse. Partridge and Pheasant, 



ACCLIM.-^Tl/INCx PAKADISE-BIRDS.— As stated in 

 "Knovvlehge" for June. 1911 (page 2371. Sir William 

 Ingram has bought the uninhabited island of Little Tobago, 

 West Indies, and is attempting to acclimatize the Greater 

 Bird of Paradise [Paradisea apoda) there. In the 

 Avictiltiiral Magazine (II, 5, 1911) he reports that he has 

 turned out forty-eight birds and that two others were to be 

 sent later. The island consists of four hundred acres of 

 forest jungle and the birds have spread over the whole area. 

 They eat fruit, insects and young birds and eggs of other 

 species. Four are known to have died, and none ha\e yet 

 mated. 



PHOTOGRAPHY. 



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



L.\TITUDE OF EXPOSURE IN LANDSCAPE 

 PHOTOGR.'\PHY. — It is well-known th.it in landscape 

 photography there is very considerable latitude in the 

 exposure which may be given, a range which is sometimes 

 ascribed to the latitude of gradation which the photographic 

 dry plate is capable of accurately rendering. Experimental 

 investigation, however, shows that the range of light intensities 

 which can be reproduced accurately by an ordinary photo- 

 graphic dry plate is only about one to twelve or one to 

 sixteen, that is to say, this is the range of intensities which 

 are included in the straight line portion of the characteristic 

 curve of a plate, or is the range of intensities comprised 

 within the period ol correct exposure of a plate. 



o 



6-4. fia ^6 -ii^ 



Intensities. 



Figure 1. 



The figure shows the " characteristic curve " of a plate, 

 that is, the relation between the light intensity acting on the 



plate during exposure and the amount of metallic silver 

 reduced by development. 



It will be seen that this curve may be divided into three 

 portions: — A portion. .AB. where the reducible silver increases 

 more rapidly than the geometrical increase of the intensities, a 

 portion, BC. where the increase rates are proportional, and a 

 portion, CD, where the increase of silver with increasing 

 exposure diminishes. 



These three portions are known : 



.4B, as the period of under-exposure, 



BC, the period of correct exposure, and 



CD, tiie period of o\'cr-exposure. 



For the curve shown the range of intensities in the period 

 iif under-exposure is about one to eight, the range in the 

 period of correct exposure eight to one hundred and twenty- 

 eight, or one to sixteen. 



Since one to fifteen is about an average range of intensities 

 for an ordinary landscape subject, it is obvious that, strictly 

 speaking, a plate has no latitude at all, that is, any deviation 

 from one fixed exposure, which we may call correct exposure, 

 will cause one end or other of the intensity scale to fall out of 

 the period of correct exposure of the plate, so that, except for 

 one particular exposure, an a\'erage landscape must have some 

 of its intensity scale rendered either within the under- or 

 over-exposed periods of the plate curve. 



In practice a plate is not considered under exposed provided 

 that any deposit at all is obtained in the deepest shadows, 

 which implies that instead of the effective range of light 

 intensities starting at the beginning of the period of correct 

 exposure, in practical photography the range of light intensities 

 starts at the beginning of the period of under-exposure, and 

 the rule that the exposure should be given for the shadows — 

 the safest of all rules in landscape photography — means that 

 the range of intensities on the resulting negative will be repre- 

 sented in both the under-exposure and correct exposure 

 periods of the plate curve. An increase of four times this 

 exposure will merely serve to shift the shadow exposure from 

 the beginning of the under-exposure region to the beginning 

 of the correct exposure region of the curve, and we may there- 

 fore expect an apparent latitude of at least one to four in 

 the direction of over-exposure. If we again increase the 

 exposure, we may shift the upper part of the scale of gradation 

 into the over-exposed portion of the curve, but for some 

 considerable distance the effect will not be appreciable to the 

 eye, and it may, therefore, be taken as a practical working rule 

 that if a given exposure is the least which can be given to get 

 the deepest shadow detail represented by the smallest possible 

 deposit on the plate, eight times that exposure will still give a 

 perfectly satisfactory negative. 



It is a matter of considerable interest to measure the 

 amount of light reflected from the shadows of landscapes by 

 means of such an instrument as the " Lumeter " referred to in 

 the May number of " Knowledge." Experiments showed 

 that in order to get a negative in which shadow detail was 

 rendered by a minimum deposit on a plate of one hundred 

 Watkins, it was necessary to give one-tenth of a second 

 exposure at FS, if the light reflected from the shadows of the 

 subject was of one hundred foot-candles intensity, and measure- 

 ment of a number of subjects showed that in bright sunlight 

 in the summer, the illumination in the shadows varied from 

 about three hundred to fifty foot-candles, according to the 

 depth of the shadow. The deduction drawn from such 

 measurements appears to lead to a complete confirmation of 

 the rule made by most experienced photographers, namely, 

 that it is difficult, with a hand camera at any rate, to over- 

 expose landscape subjects, and that a general rule when 

 dealing with a landscape in which there is a considerable 

 degree of contrast, is to give as much exposure as possible. 

 It mav be said, indeed, that one-tenth of a second at F8 will 



