6oo 



NATURE 



[February 29, 1912 



It is always present, and confined to those parts of the 

 walls of the labyrinth which are studded with warts, there 

 forming a dense carpet, which gives the dark colour to 

 the walls. The ant, Iridomyrmcx myrniecodiae, which 

 inhabits the tubers under natural conditions is a small red 

 one, but this was dispossessed by a larger black species in 

 plants under cultivation in the garden of Buitenzorg. 



The two kinds of wall-surface are thus briefly charac- 

 terised : — " One part is smooth, light brown, impervious 

 to water, free from fungus, and on which alone the ants 

 place their pupae ; the other part is warty, discoloured, 

 pervious to water, clothed with fungus, and never bears 

 pupae." Further, the ants deposit their excrement ex- 

 clusively in the fungus galleries, so that the breeding part 

 is kept pure and clean. Although a system of galleries 

 and chambers is developed under artificial conditions in- 

 dependently of ants, the association of the three organisms 

 points to a beneficial symbiosis whereby nutrition of the 

 host plant is supplemented and the ants are provided with 

 a home. W. Botting Hemsley. 



THE STUDY OF DAYLIGHT ILLUMINATION.^ 

 pROF. L. WEBER has lately published an account of 

 the series of tests of daylight illumination carried 

 out by him in Kiel during the years 1905-8. Measure- 

 ments of this kind were previously undertaken and 

 described by the author so far back as 1890 ; his main 

 object on this occasion has been to devise a more accurate 

 and convenient means of specifying daylight illumination 

 and the requisite window-area in interiors. 



The results of a new and complete series of measure- 

 ments of light from the unrestricted sky, carried out at 

 mid-day, classified for the months of the year and extend- 

 ing over the years 1905-8, are now given. The author 

 also describes an improved form of photometric apparatus 

 specially devised for this work. The results of an 

 extensive series of tests of the day-illumination in the State 

 schools at Kiel are also presented. At the time of previous 

 experiments the individual characteristics of the various 

 class-rooms and the prevailing climatic conditions had not 

 been sufficiently correlated, so that it was difficult to 

 frame very precise general recommendations. Prof. H. 

 Cohn has, however, suggested that the illumination on 

 any desk should not fall below 25 metre-candles (approx. 

 2^ foot-candles), and that this result would in general be 

 secured if the projected solid angle subtended by the 

 window-area at this desk was not less than 50 square 

 degrees. 



This solid window-angle is often taken as the sole 

 criterion of effective illumination. Yet it leaves out of 

 account the effect of reflection from the walls in the room, 

 and also the position of the window with respect to the 

 surroundings outside. 



An improvement now suggested by Prof. Weber takes 

 the form of measuring the " light-value " (Lichtgiite) of 

 the window. This quantity denotes the value of the pro- 

 jected area of the portion of the window-area which is 

 entirely unobscured by surrounding trees or buildings, 

 the area of the entire window being taken as 100. Prof. 

 Weber describes two new instruments for the convenient 

 measurement of these quantities. 



Authorities, in estimating the daylight requirements of a 

 room, usually require that the ratio of the window-area to 

 the floor-area of the room should not exceed 1:6, or in 

 some cases i : 10. The author suggests that if this ratio 

 were multiplied by the " light-value " we should get a 

 much more serviceable factor (which he denotes by P) 

 for expressing the admission of light to the room. 



Further data are needed before one can state quite 

 definitely what value P should assume for various interiors, 

 but this information could readily be obtained. As an 

 illustration the author summarises the results of tests in 

 520 typical class-rooms, the illumination on the best and 

 worst illuminated desks, and on a desk intermediate 

 between these extreme positions, being studied. For 171 

 of the rooms P had a value >io, and in 304 rooms it 

 was >5 but <;io. In conclusion, he estimates that in 

 only 5 per cent, of these class-rooms would the illumina- 



1 A reprint from the Schriften des Naturwisscnschnftlichen Vereins /fir 

 SchUs;t>ig-Hclst€in, Band xv., Heft i. 



tion, under average climatic conditions, during the ye:! 

 fall below Cohn's minimum of 25 metre-candles. 



Prof. Weber next gives an account of his examinati 

 of the conditions of illumination in the library of t' 

 University of Kiel. He shows that, so far from comp; 

 ing with Cohn's minimum figure, even the best light 

 tables would only receive 2-3 metre-candles durii 

 December. He also points out that the rule prescribii 

 the window space for a given floor-area is quite ir 

 able to rooms in which the floor is filled by vertical 

 of books, and that such shelves rarely receive su;..^. 

 light. 



Finally, there is a communication from H. Borchar 

 which contains a summary of the theoretical and e.xp^ : 

 mental methods employed for studying the distribution 

 brightness in the sky. A chart (based on a method devis 

 by Prof. Weber) is given showing the approximate intens; 

 and distribution at different periods of the year. Ti 

 sky rarely approaches the ideal diffusely radiating hen 

 sphere assuined in conventional calculations. '1 1 

 illumination is' really due to mixture of diffused and trai 

 mitted light, the proportions of which vary with differ* 

 climatic conditions. The distribution of brightness 

 accordingly. 



THE FLORA OF DAGHESTAN. 



]V/T R. N. I. KUZNETSOF concludes an article in th.- 

 ^^^ Izvestiya of the Imp. Russ. Geogr. Soc, Nos. 6-7, 

 1910, on the flora of the mountain region of Daghestan, 

 with an historical sketch of its origin and distribution. 

 Daghestan was raised above the water at the beginning 

 of the Tertiary period, and its climate subsequently became 

 drier and assumed a more continental character as the 

 Sarmatic Sea around it dried up, and consequently the 

 Tertiary forest which clothed it must . have gradually 

 dwindled. At the same time, the combined action of 

 erosion and tectonic movements produced bare slop*^- 

 which, especially those facing south, afforded excell- 

 conditions for the development of upland xerophyt 

 vegetation. Here gathered forms which had existed in 

 various parts of Daghestan from the beginning of the 

 Tertiary period, and were now distributed, some in tl 

 north, others in the south, some on the schists, others ■ 

 the limestones, and in connection with climatic condition- 



In the Glacial period Daghestan received fr-^ 

 accessions from the north, and from the west through A~ 

 Minqr. Firs and birches now clothed the country, crowd- 

 ing out what was left of the Tertiary timber trees, whirh 

 are now represented only by an occasional Tertiary birch. 

 Beiula Raddeana, or oak, Ouercus macranthera. Many 

 slopes, especially the southern, were never forested, and 

 many limestone plateaus would not harbour arboreal 

 vegetation, and there xerophytic types spread vigotjously. 



In the steppe period the forest trees retired into the 

 heart of the country, their place being taken by xerophytic 

 forms, while in the open valleys appeared representati\ 

 of the hot desert flora of the Mediterranean. The moi 

 tain xerophytic forms of Daghestan spread widely during 

 this period. Some forms, not adapted to migration, re- 

 mained in the country, others spread to other parts of the 

 Caucasus, while those easily distributed extended so far as 

 the steppes of South Russia, when these were laid bare by 

 the retreat of the Pontic Sea. Maps accompanying the 

 article show the distribution of the most characteristic 

 forms. 



VITAL EFFECTS OF RADIUM AND OTHER 



RAYS.' 

 A DOPTING the chronological order in which the radia- 

 -^*- tions of radium and other sources were discovered 

 and applied, the lecturer considered, in the first place, the 

 effects of light and radiant heat, dwelling especially upon 

 the fact that the chemical rays — i.e. blue, violet, and ultra- 

 violet — were those which had vital effects upon the 

 tissues. The differential effect of these rays as compared 

 with those of longer wave-length at the other end of the 

 1 Abstract of a discourse delivered at the Royal Institution, on February 

 2, by Sir James Mackenzie Davidson. 



NO. 2209, VOL. 88] 



