392 



KNOWLEDGE. 



October, 1913. 



focus ens with an aperture of eight millimetres (1%) in 

 diameter, it is evident that the rapidity of the lens would be 

 equal to F/32, and this would be far too small for many 

 purposes. But as the size of the aperture is limited to is of 

 an inch in diameter, the only alternative is to use a shorter 

 focus lens, and so gain rapidity in that manner. Suppose, then, 

 that we wish to obtain a rapidity equal to an aperture of 

 F/6-4; then as the rapidity of lenses varies inversely as the 

 square of their focal lengths and the square of the diameter 

 of their apertures, the desired end would be gained by 

 reducing the focal length of the lens in the same proportion 

 as the diameter of the aperture would otherwise have to be 

 increased, which in this case is five times ; therefore, instead 

 of using a ten-inch focus lens, one of two-inch focus would 



have to be employed ; and as 2 4- — = 6 • 4 we should gain the 



rapidity while the size of the aperture itself remained 

 unaltered. At the same time the photograph would require 

 to be subsequently enlarged five times in order to be seen in 

 correct perspective when viewed at a distance of ten inches. 

 If the degree of enlargement to be finally obtained has been 

 previously decided upon, then the focal length of the lens 

 necessary for use in taking the original photograph " in order 

 that the required conditions may be fulfilled " can be found 

 from the following equation : 



P-i 



m 



where F = the focal length of lens required, v = the distance 

 of distinct vision (ten inches), and in = the number of times 

 the picture is to be afterwards enlarged. Thus suppose that 

 the photograph is to be enlarged twice ; then 

 IT 10 



r = — = 5 inches, 

 and as the diameter of the aperture is to be A-inch then 

 5 "J" r = 16, and the intensity ratio would be F/16. By trans- 

 posing the above equation we are also able to obtain the 

 distance from which the enlargement should be viewed. 

 v = F (m). 



Thus suppose that a picture taken with the five-inch 

 focus lens is subsequently enlarged twelve times; then 

 v = 5 (12) = 60 inches, and the enlargement when viewed 

 from a distance of five feet should appear true to nature. 



A SIMPLE FORM OF LENS SHADE.— It is obvious that 

 any light which reaches the plate, apart from that which is 

 actually employed in the formation of the image, must of 

 necessity have an adverse effect upon it ; and it is often found 

 when using a lens larger than is necessary to cover the plate 

 that the extra light which enters the camera, not being entirely 

 absorbed by the dead black with which its interior is coated, 

 causes a certain amount of haze or mistiness to appear over 

 the entire image, which destroys somewhat the brilliancy of 

 the picture. An effect of this kind is particularly noticeable 

 when working in an unusually bright light, such as that 

 obtained by the seashore, when something that will reduce 

 this excess of light becomes desirable. Adjustable hoods have 

 been recommended for attaching to the lens, but these are 

 difficult to manage, and whatever is employed, if it act in such 

 a manner that it only allows the lens just to illuminate the 

 plate that is in use, then the rising front is rendered useless, as 

 it is necessary for the circle of illumination to be considerably 

 larger than that required to include the plate to enable this 

 adjunct to a camera to be employed at all. The writer some 

 years ago made some experiments in bright light by the sea- 

 shore, to determine as far as possible the value of lens shades, 

 using a stand camera so that the various effects could be studied 

 visually as well as photographically, and the improvement that 

 resulted in both cases when a thin piece of wood made to 

 extend some four or five inches beyond the lens, and jointed 

 so that it could be depressed to any extent short of intercept- 

 ing the rays of light, was at once apparent. A shade of this 

 description was made that could be detached instantly from 

 the camera when not required, and as it folded up it was 

 easily carried in the pocket. Such an arrangement, however, 



appears to be simply a modification of one used many years 

 ago by the late Mr. William England, and which is described 

 as a double-hinged mahogany flap which could be bent down 

 over the lens ; and from the very fact that it was found 

 valuable by this gentleman in taking the very fine photographs 

 which he produced should be an extra recommendation in its 

 favour. Then in the case of telephotography a shade in the 

 form of a hood becomes an absolute necessity for obtaining 

 brilliant pictures, since the amount of light which is actually 

 employed in the formation of the image is small compared 

 with that which enters the positive lens ; hence light traps in 

 the form of diaphragms inside the lens tube and special care 

 in the selection of a dead black and other devices are found 

 inadequate when dealing with an amount of stray light which 

 is many times greater than that used in taking the photograph. 

 On this account a tube or hood attached to the front of the 

 lens, and adjustable in length so that it can be altered to suit 

 various angles or magnifications, as suggested by Captain 

 Owen Wheeler, F.R.P.S., must be employed in cases of high 

 magnification in order to obtain the most brilliant images. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A., LL.D. 



NEST MADE BY ORANG UTAN.— A recent issue 

 (Part I, 1913) of The Proceedings of the Zoological Society 

 of London, contains an interesting photograph, by Mr. 

 Seth-Smith, of the nest made in a tree near the Apes' House 

 by the large Sumatran Orang-Utan (Simia satyrus) which 

 escaped from its cage in November, 1912. It may be recalled 

 that a careful description has been given by Moebius of the 

 nests which the Orang makes in the woods. A nest is made 

 every night or every second night. It may be a yard and a 

 half long by four-fifths of a yard broad. It consists of two 

 dozen or so branches, with loose leaves above them. It is 

 simply a temporary bed and not, as some have asserted, either 

 a hut or a nursery. 



A REMARKABLE FISH.— Messrs. Holt and Byrne have 

 described from the south-west coast of Ireland a new deep-water 

 fish (Lamprotoxus flagcllibarba) with several remarkable 

 features. The only specimen obtained had a body about seven 

 inches long, without including caudal fin and lower jaw. 

 It bore a filamentous barbel many times longer than the body. 

 The colour of the scaleless skin was velvety black, and the 

 barbel was grey. A purplish-grey cord-like band of luminous 

 tissue, partially embedded in the skin, formed a closed loop on 

 the anterior part of each side of the body. There was also a 

 large photophore behind and slightly below the eye, occluded 

 by skin save for a narrow slit. Very small photophores, 

 hardly visible externally, were present in lateral and marginal 

 series. 



HERMAPHRODITE BEES.— In a hive in the Caucasus, 

 G. Kojewnikov found hermaphrodite bees. There were also 

 normal workers and drones from the same queen-mother. 

 The hermaphrodites looked like workers with the heads of 

 drones, but there was an intimate mixture of characters. One 

 of the mandibles was a drone's, the other a worker's ; the 

 eyes were drone's eyes, the thorax was a worker's thorax. 

 The sting was very variable. An interesting point is that 

 while some of the hermaphrodites had an ovary on one side 

 and a testis on the other, others had two ovaries or two testes. 

 Yet those which were unequivocally females or males as 

 regards the essential organs of generation were, nevertheless, 

 hermaphrodites in skeletal parts. This seems to show that 

 in this case the nature of the reproductive organs does not 

 influence the development of the external sex characters. 



COMMENSALISM OF ANTS AND CATERPILLARS. 

 — Towards a hundred cases are known of caterpillars 

 (Lycaenidae and Erycinidae) living in commensalism with 

 ants. M. Charles Oberthiir has recently called attention 

 to two species in Brittany — Lycaena argiades and L. baton 

 — which appear to illustrate this kind of association. Harold 



