January, 1913. 



KNOWLEDGE. 



25 



NESTING - BOXES IN RUSSIA.— In many parts of 

 Russia, especially in the towns along the Baltic, the traveller 

 cannot help being struck by the numbers of nesting-boxes 

 in the gardens and courtyards. This is 

 particularly noticeable in the quaint old town 

 of Libau, where in almost every garden, 

 however small, a nesting-box may be seen. 

 These boxes are invariably stuck on the top 

 of a pole — never against the trunk of a tree or 

 the side of a house as with us in England. 

 In many cases a small branch is fixed to the 

 box to render the site more natural and attractive. 



This pleasing custom is due to a pious desire 

 to shelter the Dove, a bird entirely sacred to 

 the Virgin in Russia, and which is never molested 

 in any way, much less killed and used for food 

 — indeed, anyone found guilty of such an act 

 would run the risk of serious reprisals from the 

 neighbours. 



The name Dove embraces any kind of pigeon, 

 wild or tame, and fancy kinds are often kept as 

 in England. 



The owners of those boxes which happen to 

 attract a Dove are greatly envied, as the privilege 

 of harbouring and sheltering the " Virgin's Bird " 

 is considered to entail good luck. Of course, 

 only a small proportion of these boxes are 

 selected by Doves ; Starlings and other birds 

 make frequent use of them, and are always 

 welcome. 



Figure 27 is from a sketch which I made of a 

 typical box in the garden of a small house in 

 Libau. Notice the two small perches below the 

 entrance hole. Lionel E. Adams. 



PHOTOGRAPHY. 



By Edgar Senior. 



PHOTOGRAPHING ROCK SECTIONS.— One of the 

 most interesting, and at the same time valuable, applications 

 of photography to the microscope, 

 especially when considered from an 

 educational standpoint, is its use 

 in obtaining permanent records of 

 geological specimens. In work of this 

 nature, except in very few cases, 

 low powers are all that are necessary. 

 Although quartz, felspar, and mica 

 form the chief constituents in rocks, 

 the other substances which occur 

 being of secondary importance, the 

 sections themselves vary so much 

 in character that the method of 

 illumination employed in one case, 

 may be totally unsuitable in another, 

 so that no absolute rule can be laid 

 down, the operator having to use his 

 own discretion in the matter. With 

 a little experience, however, there 

 will be no difficulty in judging the 

 most suitable means to employ in 

 order to obtain the desired results. 

 With a large number of rock 

 sections it is necessary to make use 

 of polarised light in order to differ- 

 entiate their structure, and in some 

 cases selenite and mica films, used 

 either separately or in combination 

 are an advantage also. In many 

 cases the best results photo- 

 graphically are got by means of 

 the crossed nicols only ; many forms 



of granite are of this nature, as they give so much colour 

 themselves that only crossed nicols are required. Others 

 again require the selenite to show them properly. In 

 the accompanying illustration Figure 28, which is from a 



Figure 27. 



Typical Russian 

 Nesting-box. 



Figure 28. 



Section of Aberdeen Granite photographed 

 with polarized light X 70 diameters ; objec- 

 tive employed 24 mm., together with a two 

 projection ocular. 



section of Aberdeen granite, a red and green selenite 

 plate was used, in order to show the structure at its 

 best for the purpose of photography. In taking the 

 negative, a twenty-four millimetre objective was 

 used, with the analyser screwed into the 

 nosepiece above it, and a two-projection ocular 

 as eyepiece. An achromatic sub-stage con- 

 denser with its top lens removed, and the 

 polariser fitted in below completed the whole 

 optical arrangement, the source of illumination 

 being a paraffin oil lamp. In order to make a 

 correction for the so-called chemical focus of 

 the objective, the light was filtered through 

 an orange glass screen. It may not perhaps 

 be apparent why this is necessary, considering 

 the conditions under which the photographs are 

 taken, but it has been found better in practice 

 to do so, especially if lantern slides are going 

 to be made from the negatives. One very 

 essential condition above all others in order 

 to ensure success in work of this class, is 

 that the sections must be thin, otherwise 

 the definition will be very seriously impaired. 

 There should not be much difficulty in this, 

 however, with the improved methods in use 

 now for making sections. In taking the 

 photographs it is advisable to bear in mind 

 the photographic values of the colours, remem- 

 bering that they have to be translated into 

 monochrome, and that however fine the effect 

 may be when seen in colours, the result may 

 be totally disappointing in black and white. 

 It therefore becomes necessary to employ plates 

 which by falsifying the luminosities of the 

 colours themselves produce the necessary con- 

 trast in the finished photograph. This very power becomes 

 at times of immense value, as in objects which exhibit colours 

 under polarized light such as plant-hairs, cotton-fibres, silk, 

 flax, hemp, and so on, it affords a means of rendering details in 

 structure which would otherwise be lost. In preparing slides 

 for use, the objects should when- 

 ever possible be mounted in balsam, 

 as the effect is much more striking 

 than when mounted dry or in water. 

 Then, again, the Lumiere Auto- 

 chrome process affords a means of 

 producing very fine lantern slides 

 showing the colours themselves, and 

 when photographing in this way 

 the compensating screen for sub- 

 duing the too great action of the 

 blue, is not required when the oil 

 lamp is used, as the colour of the 

 light is sufficient in itself for the 

 purpose. It will also be found that 

 with moderate powers the exposure 

 is not long. 



PHYSICS. 



By Alfred C. G. Egerton, B.Sc. 



DENSITY OF LIQUIDS.— 

 While investigating the behaviour 

 of various liquids of different 

 densities which were sufficiently 

 insoluble to maintain a meniscus of 

 separation, Professor A. L. Clark 

 finds that chloroform and water 

 have equal density at a certain 

 temperature. The two liquids are 

 placed in a thick-walled capillary tube which is placed in a 

 paraffin bath. The chloroform is denser than water at the 

 ordinary temperature, but at a few degrees below the critical 

 temperature (260°) the chloroform rises to the top ; as the 



