UNDULATORY FORCES. LIGIIT. [BTFECTR ON LIVING BODIES. 



Fig. SJ. 



or lines, which cross the spectrum perpendicularly to it. 

 One of the most surprising facto connected with these 

 lines is, that their ntiml>or and position vary for every 

 kind of li_;ht. Thus, there is a set of lines for KnUr, 

 lunar, ami planetary light ; which being obtained from 

 the same independent source, the aun, ore productive 

 of identical bands ; whilat the Ijght of the fixed stars, of 

 coloured flames, <tc., each presents these band* in different 

 niiuibcrs or positions when compared with each other, or 

 with solar light. 



Bearing in mind the remarks which have been made in 

 reference to the discoveries we mentioned in the previous 

 section, the reader will not fail to perceive, that this 

 phenomenon of spectral lines is of great importance both 

 in physical and chemical researches : we shall, therefore, 

 endeavour to explain their characteristics, and to afford 

 an opportunity for the student to observe them, by de- 

 Rcribing an apparatus which will answer that purpose. 



In the annexed engraving we 

 have an illustration of the ap- 

 pearance of these dark lines, 

 as obtained from solar light. 

 We have, however, represented 

 but a small number of them, 

 because the limited size of our 

 page would not admit of a 

 greater extension. 



We are indebted for the 

 annexed cut to Sir J. F. W. 

 Herschel's article on light, in 

 the Encyclopaedia Metropoli- 

 tana ; and it affords the reader 

 a representation of the mutual 

 position and comparative fre- 

 quency of these bands in a 

 spectrum of solar light; and 

 showing, at the same time, 

 their connexion with the dif- 

 > ferent coloured rays. The 

 Roman capitals at the upper 

 part of the diagram show the 

 position of the larger and more 

 defined lines ; the italics repre- 

 sent narrower lines placed close 

 to each other. Between the 

 capital letters numerous fine 

 lines are exhibited ; and their 

 position in reference to the 

 coloured rays is indicated by 

 the names of those rays at the 

 foot of the engraving. In all, 

 about 1,000 lines have been 

 counted as existing between 

 each end of the spectrum. 



Our remarks on interference* 

 will prepare the student to un- 

 derstand the cause of these 

 lines. We there stated, that 

 when undulations of different 

 pliases, either of sound or light, 

 , met each other, or rather, 

 perhaps, united together, the 

 result would be their mutual 

 destruction. Now there is no 

 doubt that these black lines 

 arise from a somewhat similar 

 cause ; and as they show the 

 absence of light at any particu- 

 lar point where they exist, we 

 can but conclude, that certain 

 undulations which would, if 

 unimpeded, have produced 

 light in snch places, must have been destroyed, lost, or 

 absorbed from the rays incident on the refracting prism 

 during their passage from their source. 



We have already remarked that every light presents a 

 different series of these lines. In some they are compa- 

 ratively absent, or are differently placed. On reflection, 



Be* antr, p. 52. 



! 



we cannot but perceive a most astonishing instance of 

 design in that in which we could least have expected to 

 have discerned such. Every ray of light can bo identified 

 from its fellow; and there is not a greater distinction 

 between human faces in different nations than exists 

 here. The generic likeness is complete, but the indi- 

 vidual characteristic and difference are not less so. 



We have, at a previous page, t entered fully into the laws 

 of single refraction ; and also explained how the index of 

 refraction may be found for various bodies. Fraunhofer 

 applied, however, the position of the black spectral lines 

 to ascertain still more accurately the refractive indices of 

 different media. The principle of this system will be 

 easily understood, if the student bear in mind that, if 

 the rays of the same kind nf Ivjht are refracted through 

 different transparent media, then the black lines thus 

 produced by each medium differ in their distance from 

 each other according to a fixed law. Now, by comparing 

 these mutual distances in terms of each colour of the 

 spectrum, the refractive and dispersive power of any 

 body can be ascertained, and measured with mathematical 

 accuracy. 



It would, of course, be impossible for us to illustrate, 

 by means of engravings, this most interesting fact. Such 

 is entirely a work for exact experimenting, and can 

 only be conducted by those who, through long experi- 

 ence, are prepared to enter into such delicate researches. 

 Having, however, given an exposition of the principles 

 on which these investigations ax* conducted, we proceed 

 to describe the arrangement which may be employed for 

 the purpose of viewing these lines, and in measuring the 

 distances between them. 



Sir John F. W. Herschel recommends that the lines 

 should be viewed by means of a good achromatic tele- 

 scope. For tliis purpose, by means of which a magnified 

 view of the spectrum is gained, the colours obtained by 

 the use of the prism J are allowed to fall on to the object- 

 glass of the telescope. The lines, which are too close 

 together to be seen by the naked eye, being magnified, 

 may thus be easily perceived, and those in each colour of 

 the spectrum may be separately examined. In using the 

 telescope, the object-glass takes the place of the screen 

 usually employed to receive the spectrum rays. 



Mr. Ladd, of Beak-street, London, has lately manu- 

 factured an apparatus, constructed on the same princi- 

 ples as that which was employed by MM. Bunsen and 

 Kirchkoff, which can be used also to discover the pre- 

 sence of M. Fraiinhofer's lines in various spectra. It is a 

 modification of that which we have just described, and 

 will, doubtless, prove of great value as an instrument of 

 research in the new branch of what we may call Optical 

 Chemistry. 



THE EFFECTS OF LIGHT ON ORGANISED 

 BODIES. 



a former page we have noticed, generally, the effect 

 which light has on the colour of vegetable and animal 

 bodies. We here propose to enlarge on that interesting 

 subject, and to suggest some experiments which may 

 eiusily be repeated by the student. 



The most cursory observation of the appearance of 

 plants in a growing state, is sufficient to show that light 

 has a great influence on them. This may be well observed 

 in a garden whose walls run east and west. Plants grow- 

 ing on that side which is shaded from the solar rays will 

 be found generally in a sickly state, and their flowers, 

 fruits, <fec. , are few, and imperfect in their development ; 

 whilst those growing beneath the north wall, with a 

 southern aspect, and thus continually exposed to the full 

 rays of the sun, reach their proper maturity and size. 



We may also notice that plants, trees, <fcc. , invariably 

 shoot out more branches on the side on which they are 

 exposed to the sun. This may be well illustrated by 

 placing a geranium or other quick-growing plant in a 

 window with a southern aspect, and retaining it in one 

 position for two or three weeks. It will be found that 

 numerous shoots spring out towards the window ; whilst 



1 S aitlt, pp. 49, 4. Antr, p. 4. I Ante, p. 50. 



