March 16, 1883.] 



KNOWLEDGE 



161 



was aJso shown by the nucleus. In this t}ie dark lines 

 of Fraunhofer were not conspicuous, if visible at all, 

 showing that the principal brilliancy of the comet was not 

 redacted sunlight The usual carbon bands of the conietary 

 spectrum were not visible through the sky illumination, 

 and no other bright lines except those of sodium were seen 

 by Thollon. On the 22nd the comet's spectrum was 

 observed in the early morning just before sunrise, by 

 Ricco, of Palermo. He reports iiis observation thus :— 

 ■"The spectrum was formed of the narrow continuous 

 spectrum of the nucleus, traversed by a large and strong 

 line of sodium (D) ; by enlarging the slit I saw a globular^ 

 monochromatic image of the nucleus and coma. Besides 

 the line of sodium, many others were present, but, my 

 spectroscope not having a micrometer, I did not determine 

 them. I observed a band in the red, a line in the yellow 

 near and after D, two others in the green, and an enlarge- 

 ment of the continuous spectrum in the green and blue." 

 It is exceedingly unfortunate that the position of these 

 lines could not have been determined, at least approxi- 

 mately. No one can predict when such an opportunity 

 •will occur again. 



The weather in this part of the country was abominable 

 up to November. I attempted to get spectroscopic 

 •observations on Sept. 20, but was foiled by clouds, 

 and have since succeeded only on October 2, 4, 10, 15, and 

 -•*• On the first of these dates the sodium-lines were still 

 ■easily visible, though not conspicuous. The carbon bands 

 were magnificent, especially the brightest one (in the green), 

 in which could be clearly seen the three fine lines observed 

 in the spectrum of Coggia's comet The band in the violet 

 ■*'as verj' faint The nucleus gave a strong continuous 

 spectrum, on which the carbon bands wore superposed ; 

 and in the tail the proportion of white light (continuous 

 spectrum) to carbon light appeared to be about the same 

 as in the nucleus. The bands could be followed far out 

 into the tail by widening the slit, but were lost before the 

 continuous spectrum quite vanished. No dark lines were 

 made out On the 4th the results were the same, e.xcept 

 that the sodium-lines were very hard to see, and they dis- 

 appeared entirely before the next date. The later obser- 

 vations added nothing more. It is much to be hoped that, 

 when the different results of all observers come to be col- 

 lected and published, something will be found to supply 

 what is so unfortunately wanting in Ricco's most interests 

 tng but incomplete observation — hialxs valde dejfendus. 



The highest interest of tho present comet lies in its 

 •orbit, however, its relation to preceding comets, and its 

 possible speedy destruction by the sun. Almost as soon as 

 it appeared, Professor Boss, in America, and Hind, in 

 England, proposed the hypothesis that it is identical with 

 the great comet of 1880, the period of the latter comet 

 having been shortened by some resistance. If so, this 

 comet will lie back again in a few months, and before long 

 must fall upon the sun. They have weighty arguments on 

 their side, but on the whole a different conclusion is more 

 probable. 



(To he continued.) 



AsBEsriDs rope is described amongst other articles in a new 

 •catalogiie pnblisbed by the United Asbestos Company. The 

 strength seems to be about one-fonrth that of ordinary hemp rope 

 ■of the same diameter. Rope 1'5 in. diameter haa a breaking 

 strength of one ton, and 20 ft. of it weighs 131 lb. The breaking 

 strength of the rope 0'6875in. diameter — \'„ — is 02 tons, or 4cwt., 

 a 20 ft. length weighing 3 J lb. The rope is made especially for fire- 

 escapes purposes, for theatres, fire brigades, and for ready means 

 •of escape from liouses and public buildings, the advantage being 

 that the rope will not break and drop its burden if a flame bears 

 upon it. It is made like ordinary rope, but spun from Italian 

 ■Asbestos thread. 



HOW TO USE OUR EYES.— III. 

 Bv John Bbowning, F.R.A.S. 



(Continued from p. 150.) 

 COMI'LEMEKTARV COLOURS. 



^T^HEN a coloured object is looked at for some time, 

 * * if the eye be directed to another object which is 

 grey or colourless, this object will appear to be strongly 

 coloured, just the opposite colour to that of the first object 

 the eye had been regarding. 



By the expression the opposite colour, I mean the com- 

 plementary colour — that is, the colour which added to the 

 first colour would produce white. 



Blue causes the grey portion of the object to appear 

 yellow, while yellow causes the grey portion to look blue. 

 Red causes the grey to appear green, while green makes 

 the grey look red. 



It must not be supposed that if green paint be mixed 

 with red, or orange paint with blue, that white will be 

 the result 



Owing to the impurity in an optical sense of all our 

 colours, we obtain only dark greys by such mixtures. 

 But if you reflect the complementary colours from the 

 spectrum, in which the colour is optically pure, one on the 

 other, 3'ou can make colours that will be very different to those 

 you will obtain from the mixture of pigments, and by using 

 carefully-selected pieces of coloured glass in two lanterns, 

 colours will be produced very different to those obtained 

 by the mixture of pigments. 



The colours of stained glass are much purer and brighter 

 than any of our pigments. 



There are three ways by which colours can be mixed. 

 1st, by grinding up the colours together. 2nd, by laying 

 coats of colour over each other. 3rd, by making narrow 

 lines or dots of the colours close to each other. The two 

 last methods are but little known or used, yet they are 

 the methods which give the most beautiful as well as the 

 most scientific results. 



On a revolvin;/ wheel place a disc covered with black 

 and yellow paint mixed together. The disc appears green. 

 On another disc let a portion of the surface be coloured 

 yellow and a portion black. On rotating tliis disc the 

 colour will be not dark green, but dark yellow. Jli.x blue 

 and red, and then blue and yellow, first by mixing the 

 paints, and then by colouring a portion of each disc, and 

 note the different result obtained by the two methods. 



The diagrams invented by Mr. Gorhani, the inventor 

 of the coloured top, will show the appearances I have 

 just described. You will see the grey portion in the 

 blue disc appear yellow, the grey portion of the red disc 

 appear green, and the grey portion of the green disc 

 appear a reddish chocolate. 



It is from want of knowledge of this fact that many 

 artists over-colour their pictures. They make their 

 shadows too blue in what they would call a warm picture 

 • — that is, a picture of a red or orange or yellow tone of 

 colour, and the shadows in a cold or bluish picture they 

 make too red. This in another direction proves the 

 necessity of learning to see. Such artists require to be 

 shown that perfectly colourless shadows in a Muish picture 

 will look red, and equally colourless shadows in a reddish 

 picture will appear blue. 



Although we cannot with our unassisted eyes tell how- 

 pure or impure colours or paints are, we can detect their 

 impurities by means of a spectroscope. 



There are many coloured liquids and glasses which look 

 to us almost exactly alike. Now let us see how they look 

 when we analyse or cross-question them by sending the 



