July 1, 1895. 



KNOWLEDGE. 



14-9 



much more deadly character have made their appearauce. 

 Of the many canes under cultivation in the West Indies, 

 the variety usually grown, the Otaheite or Bourbon, is 

 much more severely attacked than some others. There 

 are what appear to be " immune" canes; and one imme- 

 diate remedy, which has been extensively adopted in St. 

 Kitts and Barbados, is the substitution for the Otaheite 

 of some such cane as the Caledonian (^)ueen. 



A great stride has been taken during recent years in 

 another direction. The canes have been propagated from 

 time immemorial by vegetative means, i.e., by putting in 

 pieces of the parent plant, so much so that the fact had 

 apparently been lost sight of that the seed of the arrow is 

 occasionally fertile ( Fig. 5). The discovery of the possibility 

 of raising canes from seed is a matter of recent years ; and it 

 is hoped by this means to reconstitute the plant, to impart 

 to it a vigour and capacity for resisting disease, and at the 

 same time to increase its sugar-producing properties. 



Bat the path is long and laborious ; the obtaining of 

 fertile seed by the crossing of varieties is a matter of no 

 ordinary difficulty, and, once obtained, two years must be 

 passed through before the first canes can be reaped and 

 tested. The vast majority of seedlings thus raised are of 

 comparatively little value, and it is only b ". ; and there 

 that one is found which is considered worthy of further 

 trials. Then it has to run the gauntlet of diseases, besides 

 entering into competition with the most approved sugar- 

 producing varieties — the products of selection acting 

 through twenty centuries. Trials have, however, been 

 conducted in many places — .Java, Mauritius, Barbados, 

 and Demerara — and the laborious work of Prof. Harrison 

 in the two last-named colonies is a monument of patient 

 and scientific application. With continued experiments 

 of this character, an improvement is certain, judging by 

 what has been done in other cultivations by seminal selec- 

 tion ; but the tedious nature of the operation will of itself 

 serve to deter other colonies, less fully equipped with the 

 means of carrying on the experiments, from proceeding 

 with this important work. 



SPECTRUM ANALYSIS.-I. 



By J. J. Stewakt, B.A.Cantab., B.Sc.Loud. 



WHEN sunlight passes through a prism or wedge- 

 shaped piece of transparent substance it is 

 bent out of its course ; and if the source 

 of light is a narrow slit — for example, if the 

 light from the sun is made to pass through 

 a small opening in a shutter, the beam of light is spread 

 out after passing through the prism, and if it is received 

 on a screen the image of the slit will consist of a broad 

 band of various colours, red at one end and blue or violet 

 at the other. This bright and many-hued strip is the 

 solar spectrum, and its origin is due to the fact that the 

 amount of bending or refraction which takes place on the 

 passage of light through a prism depends on the colour of 

 the light. Blue Ught is more refrangible than red, i.e., it 

 is farther bent from its original course, and, therefore, 

 shines on a difi'erent part of the screen from that which is 

 illuminated by the red rays. The splitting up of the rays 

 which occurs on the transmission of light through a prism 

 shows that white light is of a composite nature — that is 

 to say, it is made up of rays which differ in colour and 

 refrangibihty. 



If sunlight be admitted through a small hole in a shutter, 

 so as to fall on a prism placed with its refracting edge 

 horizontal and its base uppermost, the light will be refracted 

 and bent upwards, and may be received on a screen placed 



at a distance. Now, if the prism is gradually turned round 

 an axis passing through the centre of the prism, and 

 parallel to the refracting edge, so that the coloured spectrum 

 produced is more and more bent downwards, and moves 

 down the screen as the prism is turned, it will be found 

 that at a certain point in the rotation the spectrum 

 becomes stationary ; and on continuing the rotation of the 

 prism in the same direction, the image begins to ascend 

 and gets higher and higher on the screen. The position of 

 the prism when, after descending, the coloured image 

 of the sun turns and begins to go upward, is called the 

 position of uiiiiiunuii iln-iatiou. For that particular position 

 the rays are least deviated from their original co\irse, and 

 they make equal angles with the faces of the prism at 

 incidence and at emergence — iu other words, the rays pass 

 symmetrically through the prism. 



That white light is made up of rays of different colour 

 and refningibility was first proved by Sir Isaac Newton. 

 He also showed that lights which differ in colour differ also 

 in refrangibihty. By painting an oblong piece of black 

 paper with red on one half and blue on the other, and then 

 viewing the paper when thus coloured through a prism, he 

 found that the light reflected from the blue half was more 

 refracted than that from the red. Placing the paper 

 opposite a window, and looking at it through a prism with 

 its refracting angle downwards, so that the rays were bent 

 upwards, he observed that the blue was lifted higher than 

 the red. When the angle of the prism was pointed 

 upwards the rays were deflected downwards, and the blue 

 were bent lower than the red. In both positions the light 

 from the blue half of the paper was more refracted than 

 that from the red. 



The prismatic spectrum is now observed and investigated 

 by means of the spectroscope. Light is allowed to fall 

 on a narrow slit placed at the end of a telescope tube ; at 

 the other end of the tube an achromatic lens is fixed at a 

 distance from the slit which is equal to its focal length, so 

 that when the light leaves the lens it consists of a bundle 

 of parallel rays. This part of the apparatus is called a 

 collimator. On a movable horizontal table a prism is 

 placed, and the parallel rays from the collimator are made 

 to fall upon it. They are thus refracted and are then 

 observed through a telescope. The spectrum is brought 

 to a focus by the object glass of the observing telescope, 

 and is then viewed through the eye-piece. Both the 

 collimator and the telescope are attached to a graduated 

 circle, and the telescope is capable of motion round this ; it 

 can be fixed in any desired position, and this position can 

 then be read off on verniers attached to the telescope and 

 moving round on the graduated circle. The spreading out 

 of a beam of light into a coloured band, owmg to the 

 varying refrangibilities of the component rays, is called 

 dispersion. As in the spectroscope the rays leave the 

 collimator parallel to each other, they are dispersed in 

 the prism, and on leaving the prism there is a beam of red 

 hght with a beam of blue Ught at a distance from it, the 

 red rays being parallel to each other, though inclined to 

 the blue. The yellow and green rays come between, the 

 rays of each colour being parallel amongst themselves. 

 The red rays are thus brought to a focus by the observing 

 telescope at a definite point, and the blue rays at a 

 neighbouring but different point. Thus a pure spectrum 

 may be obtained — that is, one in which the different 

 colours do not overlap. The position of minimum 

 deviation is found in this instrument by turning the prism 

 round by moving its stand till the light is less and less 

 deviated, and following it round with the telescope, when 

 at length a position is reached in which, on further move- 

 ment of the prism, the image of the spectrum in the 



