NATURE 



[February 13, 19c 8 



nuuital study (pp. lo— 16) by Dr. Shrub.'iall of the statistics 

 of growth. Tlio general results might with advantage 

 Iiave been further elaborated on the practical side. As lo 

 Hiih, there is a strong plea for school dental clinics on the 



iiodtl of Strassburg. 

 A special investigation as to tuberculosis of the lungs in 



Jiool children was undertaken by Dr. Squire and Dr. 

 Annie Gowdey. Of actual phthisis, only 335 cases (i.e. 

 <i-55 per cent.) were found among 58,934 children. The 

 sections on hearing and acuity of vision contain much fresh 

 material. One of the most important sections deals with 

 the " development of articulatory capacity for consonantal 

 suunds " (p. 27). Considerable detail is given of the 

 niithods of testing, and 105,000 tests were made on some 

 3000 children. The results are given in an exact quantita- 

 tive way, capable of analytical study. This department is 

 of immense importance to the teacher, as the work already 

 done in phonetics has abundantly shown. As to fatigue, 

 same new curves from rifle-shooting are given. It is 

 found that the curve improves with a little practice, co- 

 ordination improving very rapidly. Cigarette-smoking was 

 found to impair the capacity to shoot straight. 



There are the usual sections dealing with the inspection 

 of defective children and cripples, country homes, infectious 

 diseases, adenoids, &c. ; but two sections must be speciallv 

 ii.-uned, one on the artificial lighting of school-rooms and 

 ihe other on the mental and physical effects of bad ventila- 

 tion. In both researches the practical results are very 

 definite, and ought to be driven home among teachers and 

 architects alike. Of the ventilation research, some pro- 

 visional conclusions arc : — " Temperatures above 65° \' . 

 give rise to definite subjective symptoms, slackness and in- 

 attention in some, headaches in others. Although it is 

 not easy to assert definite mental alteration till about 

 to" F." " Symptoins do not appear at 65° if the air is 

 kept in gentle movement by a fan in the room. With 

 temperatures 70° F. and above, other factors being normal, 

 Ihere are marked symptoms and very evident deterioration 

 in mental alertness and accuracy." At low temperatures, 

 relative humidity does not affect the mental capacity of 

 children, but increase of humidity increases the effects of 

 high temperatures. C.-irbonic acid gas in considerable 

 excess increases markedlv flie fatigue of the children. 

 Kxact details are given of the methods used. 



The London County Council is to be congratulated on 

 the issue of this mass of original and important observ.i- 

 tions in so many departments of medical inspection. Dr. 

 Kerr's reports show the great educational possibilities of 

 the system, which, under his guidance, has revealed manv 

 new regions for clinical and scientific research. 



THEORY OF THE MIRAGE. 



'T'HE theory of the mirage forms the subject of several 

 recent papers by Prof. Antonio Garbasso. In notes 

 contributed to the Aui dci Lined, xvi. (2), i, 8, the 

 author discusses the propagation of light in a hetero- 

 geneous medium, making use of the principle of least time, 

 and considering the case of space of any number of 

 dimensions defined by curvilinear coordinate's. The space 

 in question is supposed to be subject to the usual assump- 

 tion that the square of the line-element is a homogeneous 

 nuadratic function of the differentials of the coordinates. 

 .\s might be expected from the principle of least action (an 

 .analogy the applications of which to the problem are prob- 

 .ably already known), the equations of the path can be re- 

 duced to the form of the ordinary equations of dynamics 

 by a suitable choice of the characteristic function. The 

 applications to the mirage itself are discussed in a paper in 

 tlie Memoric of the Turin .Academy, 1907. Prof. Garbasso 

 liims that while the phenomenon has been studied both 

 ^perimentally and theoretically, his present work fills a 

 ip in the literature by establishing agreement of a 

 quantitative character between the results of calculation 

 and those of experiment. 



Two kinds of mirage are distinguished, one due to the 

 variations of density caused by diffusion between two 

 lluids of different refrangibilitv ' initially h.aving a plane 

 of separation ; this is called the mirage of Vince. The 

 second kind, called the mirage of Monge, depends on 



NO. IQ98, VOL. yy] 



diffusion outwards from a plane boundary maintained in- 

 definitely at the same conditions. The former condition 

 gives three images, two direct and one inverted ; the latter 

 gives only the reflected image. Prof. Garbasso calculates 

 toe law of density from the equations of diffusion, and thus 

 determines the equations of the trajectories of the rays of 

 light and the form of the wave-front. 



Ihe final comparison with experiment is discussed in a 

 paper by Luigi Rolla, also in the Memorie of the Turin 

 -Academy. In it the last-named author describes experi- 

 ments showing how, not only has Wollaston's original 

 artificial mirage of the \'ince type been reproduced with 

 its three images, but also the Monge mirage has been 

 imitated, and in both cases the trajectories of the rays 

 have been determined by observation and compared with 

 results of theory. Moreover, a mirage with five images, 

 observed by Parnell at Folkestone in 1869, was realised 

 by placing over a layer of carbon bisulphide a mixture of 

 equal parts by volume of alcohol and chloroform. Owing 

 to the unequal rates of diffusion, the conditions give rise 

 to five images, and this and other experiments are shown 

 to be suitable for lecture-room demonstration. 



By taking a block of gelatin containing a cavity filled 

 with liquid which gradually diffuses into the gelatin, the 

 corresponding images for a cylindrical or spherical dis- 

 tribution of density have been also produced and compared 

 with the results of mathematical calculation. 



The first and second figures show the mirages of a 

 di.iphragm somewhat in the shape of a ship produced by 

 lln- medium formed by diffusion between alcohol an. I 



FlG. 



bisulphide of carbon. Fig. i represents the appearance 

 after a few hours. Fig. 2 after several days. Fig. 3 shows 

 the five images obtained by diffusion between bisulphide of 

 carbon and a mixture of alcohol and chloroform. 



G. H. B. 



.4 CONTRIBUTION TO THE HISTORY OF 



IRONCLADS. 

 T ORD ROSSE has made an interesting contribution to 

 the history of ironclads by placing at the service of 

 the Institution of Naval Architects copies of letters written 

 by his father to various distinguished men in the years 

 1854-5. From these letters it appears that the late Lord 

 Rosse not merely appreciated the importance of armour 

 protection against horizontal shell fire, but satisfied him- 

 self that it was possible by means of suitable proportions 

 to secure ample stability in ironclad ships. Naval officers 

 were then disposed to think that the " top-weight " in- 

 evitable with heavy loads of armour would make vessels 

 unstable. Lord Rosse proposed the construction of iron- 

 clad floating batteries of moderate size ; they were intended 

 to fight in smooth water, and consequently were to carry 

 their guns at a small height above water. The exposed 

 sides were to be armoured with 5 inches of iron, and the 

 upper decks to be covered with 2-inch plating. 



In a letter to Sir John Burgoyne dated June 26, 1854, 

 Lord Rosse proposed an armament of sixteen heavy guns ; 

 the draught of water was not to exceed 12 feet to 13 feet, 

 .and the vessel was estimated to be about 1500 tons. He 



