462 



KNOWLEDGE. 



NO\EMBER, 1910. 



REPLIES. 



7. SCIENTIFIC IDEAS OF TO-DAV.— There are 

 probably many readers who will sympathise with the ditficulty 

 e.\perienced by your correspondent " Perplexed." The truth 

 at the present time is that hardly anything is known for 

 certain about the formations of the radiating systems 

 associated with the atoms and molecules of a body reflecting 

 light. 



The complexity of optical phenomena calls for the detailed 

 investigation of special cases before it is possible to do more 

 than generalise. The problems of radiation are now in the 

 process of solution, and such notable advances as the discovery 

 of the Zeeman effect are gradually throwing some light on the 

 question of the forms of orbital or other motion performed by 

 the radiating electrons ; but there is a very great deal to be 

 done before any definite statement of what occurs in special 

 cases can be made with safety — until then only the most 

 comprehensive generalisations are possible. 



The formation of mental pictures of the processes in\ olved 

 in the radiation and reflection of light is a great help to under- 

 standing them, but, in the present state of our knowledge, such 

 pictures must not be too definite and rigid ; there must be 

 ample room for modification. I am sure that the author of 

 Scientific Ideas of To-Day. will agree with me that we 

 cannot yet form any adequate idea of the motions performed 

 by the electric charges in a body which is reflecting light. 

 The book in question, being a popular treatise, contains some 

 forceful, and. I think, skilful descriptions and analogies. The 

 temptation in writing such a work is probably towards too 

 definite a form of statement, but where emphasis and 

 simplicity of illustration are essential in order to make the 

 subject as clear as possible to readers who have no special 

 scientific training, it is hard to see how this can be altogether 

 avoided. Personally, I would congratulate the author on his 

 generally successful attempt to accomplish an admittedly 

 difficult task. 



To turn to the special difficult^' mentioned by your 

 correspondent, I will point out that in the case of solids we 

 have to deal with more or less complex molecular aggregates. 

 The effect of increased temperature in many cases is to 

 modify and partialK- break down such groupings into simpler 

 forms. The selective reflection of incident waves of any 

 particular frequency seems to depend on what may be termed 

 the " laxity " of the electron system in the reflecting body. 



Stated somewhat crudely, the question becomes one of the 



natural period of oscillation of the electric charges within the 

 complicated molecular groupings as related to the periods of 

 the mixed incident radiation. If, now, we break down or 

 otherwise modify these molecular aggregations, the " tuning " 

 of the whole system is altered, and the freedom of excursion 

 of the receptive electrons is affected either on the side of 

 increased or decreased frequency. We do not know in such 

 cases the distribution and subsequent redistribution of the 

 restraining forces which come into play in controlling the 

 movements of the electrons, and the acquisition of such 

 knowledge is one of the problems for future science to 

 endeavour to solve. In the case under consideration — the 

 double iodide of mercury and silver — I would put the matter 

 thus: — Increase of temperature to about 110° F. results in 

 partial dissociation of the molecular groupings which were 

 characteristic of this compound at normal temperatures. As 

 a result, a different electron system or arrangement is instru- 

 mental in the selective reflection of light, in this case the 

 period being longer. 



I \euture to think that we can form a more definite concep- 

 tion of the probable interior structure of an atom as made 

 evident by the phenomena of radio-activity, than we can of 

 that particular radiating portion of it which is responsible for 

 the emission of light-waves, or of the complex groupings of the 

 electrons bound up in molecular aggregates on which selective 

 absorption and reflection depend. The electro-magnetic 

 theory tells us this: — That light (and similar radiation) is due 

 to electro-magnetic disturbances, periodic in space and time, 

 propagated in the ether of space, and that these waves 

 are originated by the oscillation of electric charges about a 

 mean position, or position of equilibrium within the radiating 

 body. From such a broad generalisation, science is proceed- 

 ing to the investigation of special cases. A host of facts have 

 to be embraced in any future theory of optics which claims an 

 approach to completeness, but the work already done is a 

 splendid testimony to the zeal and skill with which the subject 

 is being investigated, and to the success with which the 

 enquiry has so far been attended. 



Chari.es N\'. Kaffetv. F.R.A.S. 



10. \VAT1:K and ITS own level,— Things are said 

 to be on the same level when they are equally distant from the 

 centre of attraction of the earth. The radius of the earth 

 is so large that such things appear to lie in a plane instead of 

 on a curved surface, and hence a small surface of water appears 

 to be flat, and the curvature is only seen on a large extent of 

 surface, such as the ocean. 



A. T. 



NOTICES. 



A PICTORIAL BAROGRAPH CHART.— Mr. John 

 Browning has published a new form of chart for the barograph. 

 It is printed in colours, and pictures of clouds have been 

 introduced with such pleasing effect that a greater interest 

 will, no doubt, be taken in barometrical records if made with 

 its llelp. 



BRUSSELS EXHIBITION AWARDS.— Among the 

 scientific instrument makers who have received awards at the 

 Brussels International Exhibition we notice that Messrs. 

 Adam Hilger, Ltd., obtained a grand prix for spectroscopes 

 and spectroscopic apparatus, and Mr. J. H. Steward a grand 

 prix and gold medal for surveying and militai-y instruments. 



THE CAMBRIDGE POCKET DIARY.- We have received 

 a useful little diary from the Cambridge University Press, 

 which covers the period of the academical year, nameh- from 

 September 20th, 1910, to the end of December. 1911. On 

 one side of the page information useful to members of the 

 University is given, and the price in roan, limp, with gilt edges, 

 is one shiUing. A larger diary is also published, each sheet 

 of which, measuring ten inches by eight inches, contains 

 seven days, and the price is one. shilling net. 



NATURE-STUDY LANTERN SLIDES.— We are pleased 

 to notice the series of slides made by Mr. Holmes, of Rochester, 

 of which he has submitted a number to us. They illustrate 

 trees and their life-history, mammals and birds' nests and 

 many British plants. The slides are really excellently turned 

 out and the only criticism that we may offer is that the parts 

 of plants, at any rate, are too large, and remind one of crowded 

 plates rather than nature photographs. We would suggest 

 that the effect would be better, and greater justice done to the 

 photographs if a little more margin were left. 



LEWIS'S CIRCUL.ATING SCIENTIFIC LIBRARY.— 

 We have received from Mr. H. K. Lewis, 136, Gower Street, 

 W.C., a copy of the list of new books and new editions added 

 to the library during July, August, and September, and find, as 

 usual, that it includes every book of any importance published 

 during that period. We ha\e previously called the attention 

 of readers of " Knowledge " to the great utility of this library, 

 providing access, as it does, to all books on scientific subjects 

 for a comparatively nominal subscription. The list also affords 

 a \aluable means of selecting scientific books for purchase, as 

 it gives full particulars of each one, together with the published 

 price and postage. It is sent post free on application. 



