Xi)\i;mi!ER, 1911. 



KNOWLEDGE. 



433 



(cl \\'hat is the formulae to obtain the lieHographic 

 longitude of the centre of the Sun's disc ? 



Also : — Is there any way that I could apply the difference 

 in time between Greenwich and Sydney to the tables of the 

 Xaiitical Almanac, pages 513 and so on, showinj; position of 

 Jupiter's satellites, so that the\- would be shown as they would 

 appear at Sydney ? 



In what publication lor iiublicationsi could I find the 

 formulae known as " Lcuschner's Slimt Method " of comput- 

 ing comet orbits ? 



CKNTAUKL'S.- 



ANSWERS. 



44. COLOURS OF THE SPECTRUM.— The green p.art 

 of the spectrum cannot be divided into yellow and blue ; other- 

 wise, as the querist suggests, the prism would fail to produce 

 a green band. The only effect of using a prism to split up the 

 green part would be to magnify that part, as it were, by show- 

 ing more gradations of green. 



The term " Primary Colour " does not mean " A colour 

 incapable of analysis into others"; the "Primary Colours" 

 are those sensations out of which all other colour-sensations 

 are built, according to Helmholt^' theory. For instance, 

 though yellow light may have a definile wave-length, when it 

 strikes the retina three distinct sets of nerves respond, in such 

 a proportion as to give the sensation of yellow-. Other colours 

 excite them simply in different proportions, and the colours 

 which excite only one set of nerves at a time are called the 

 Primaries. They are a certain red, green and blue. But, 

 although the sensation of yellow is made up of a mixture of 

 three sensations, it does not follow that yellow liglit can be 



split up into three differently coloiu'ed lights. 



C. N. F. 



44. COLOURS OF THE SPECTRUM.— The green 

 region of the spectrum cannot be resolved into yellow and 

 blue. Every part of a continuous spectrum has its own 

 particular wave-length, and any very narrow portion of it may 

 be considered as appro.ximately monochromatic. The colours 

 of the spectrum, when produced under proper conditions, 

 (such as with a narrow slit, and so on), are pure ; and it is 

 important to distinguish clearly between such colours and the 

 subjective tints which can be producd to imitate them by the 

 proper blending of rays of other colours. On page ten of the 

 new edition of Professor R. Wood's treati.se " Phj'sical 

 Optics," there is the following I'eference to subjective colour : 

 " The colour depends upon the wa\e-length, but colour caimot 

 always be taken as an indication of wave-length, as ceitain 

 colours can be imitated by the simultaneous action upon the 

 retina of two trains of waves, either of which acting alone 

 would give rise to a totally different colour from that 

 perceived when both act together." [The italics are mine] . 

 " For example, a yellow scarcely distinguishable from the 

 yellow of the sodium flame, can be produced by a mixture of 

 red and green light in proper prc)|)ortions." 



Ch.arles W. Rahi;tv. 



45. HEAT AND A VACUUM.— Heat can traverse a 



vacuum in the form of radiant heat, which has the property 



of being reflected by polished surfaces ; thus the heat emitted 



from the inner vessel in a vacuum flask is sent back by the 



inner surface (polished) of the outer case; of course, some 



may be again reflected, back and forward, and it is to be 



remembered that at each reflection there is a slight loss ; but 



at any rate, all the heat emitted is not lost, only that which is 



reflected an even number of times and then absorbed. If the 



space contained air, however, the conducting power of the air 



layer would carry heat to the case, gi\-e it up to the case, and 



so cause a continual loss, ,- ,. ,, 



L . N. r . 



50. (31 RADIO-ACTI\TTV.— It is generally considered 

 that the glow ("luminescence") of the glow-worm is due to a 

 process of oxidation. The light-giving organs consist of two 

 layers. The inner one is of an opaque whiteness, and is 

 possibly protective and reflecting; the outer is semi-transparent 

 and of a slight yellow colour. 



Between the two layers is a network of very fine trachae, 

 which, it is presumed, supply the necessary air to the substances 

 the oxidation of which results in the production of light. 



The light of many Lampycidae. especiall3' those out here, is 

 regularly intermittent, i.e., the light appears for, say, two 

 flashes, and then ceases for the time of four flashes of equal 

 duration; others are three to one : but the causal mechanism 

 is, I beheve, unknown. l. G. Gilpin-Brown. 



50. RADIO-ACTIVITY.— (1) It is now accepted that the 

 a particles emitted by radio-active elements are atoms of the 

 gas helium. In the case of niton (radium emanation) this is 

 capable of spectroscopic verification. 



Such processes may be regarded as spontaneous trans- 

 formations of substances which we ha%e every reason to 

 consider as elements. 



(2) The X-rays and the 7 rays from radio-active substances 

 may be considered as closely similar types of radiation the 

 actual nature of which is very uncertain. Many eminent 

 physicists favour a pulse theory, — i.e.. one which supposes 

 the radiation to be composed of very thin pulse-shells in 

 which the wave-front is not uniform but nucleated ; and that 

 such pulses are due to the negative accelerations (X-rays), and 

 the positive accelerations (7 rays) of swiftly moving electrons. 

 Other investigators favour the view that both types are 

 corpuscular, being composed of neutral pairs, and hence 

 not deviated by magnetic or electrostatic fields. We may 

 reasonably hope for more definite knowledge in the near 

 future. 



(3) The light of the " glow-worm " is believed to be due to 

 the active oxidation of a peculiar photogenic substance pro- 

 duced by the insect. Experiments support this theory. 



Ch.arles W. Raffktv. 



REVIEWS. 



CHEMISTRY. 



Chemistry lu the New Edition of the Encyclopaedia 

 Britanuica. 

 The requirements of an article upon a particular science in 

 a general encyclopaedia are. firstly, that it shall be easily 

 understood by an intelligent reader who is practically 

 unacquainted with the subject; and secondly, that it shall 

 contain a full bibliography to which the expert can refer for 

 directions where to find what he wants. The chemist, for 

 example, will not want to use the Enc\clopaedia as a text 

 book, while the average general reader will be repelled b\- 

 pages of mathematical formulae which are appropriate enough 



in the text book. -Any attempt to satisf\- both classes of 

 reader will inevitably fail to satisfy either. 



Judged by this criterion, the new edition of the 

 " Encyclopaedia Britannica," marks a great advance upon the 

 previous edition. As instances of the way in which the 

 chemical articles meet these requirements, we may cite those 

 upon " Elements," " Colour " and " Fluorescence," which any 

 educated person can follow. The general article upon 

 "■ Chemistry." admirabl>' written as it is, errs somewhat in the 

 direction of the text book. 



In most instances, the subject matter has been brought well 

 up to date. For example, the latest details on metallography 



