SECTION II. 

 LIGHT, AND ITS APPLICATIONS, 



CHAPTER I. 

 OPTICS, AND OPTICAL INSTRUMENTS. 



THE phenomena which depend on the agency of light, 

 have been ranged under the branch of science called 

 Optics ; and we shall now have to consider this most in- 

 teresting and fascinating subject. 



Our Introductory Chapter on the Undulatory Forces, 

 and the remarks which we made thereon in our section 

 on heat, will have, to a considerable extent, prepared 

 the reader for a more enlarged view of their laws and 



Zlications in respect to light. We are, indeed, en- 

 id to measure both the number and extent of lumi- 

 nous undulations with great accuracy; and thus shall 

 have the satisfaction of dealing with substantial truth, 

 instead of vague generalisations. 



The chief source of light is the sun ; and, as we have 

 already remarked, the solar luminous rays are always 

 accompanied by calorific rays, which can, by certain 

 means, be separated from each other. Light, again, like 

 heat, is found in a latent state in all bodies ; and by 

 mechanical, electrical, and chemical means, can easily be 

 brought out so as to be perceived by the senses. 



The light afforded us by the sun, is called white light ; 

 but, by a very simple arrangement, it can be decomposed, 

 and resolved into numerous rays of various colours. 

 For this purpose a prism is employed, which is a piece 

 of glass, of a triangular shape ; and on a ray of white 

 light being allowed to pass through it, the component 

 rays are easily separated from each other, and may be 

 received on a sheet of paper, for complete examination. 



The annexed figure illustrates the most convenient 

 mode of carrying out this experiment. 

 Fi. 1. 



A ray of sun-light (a) is allowed to pass into a darkened 

 room, through a hole in a shutter (6), and to impinge on a 

 glass prism (e). The light, on emerging from the opposite 

 side of the prism, will be divided into numerous rays, 

 having at least seven different colours ; namely, red, 

 orange, yellow, green, blue, indigo, and violet They 

 are observed in the order in which we have arranged 

 them ; the red being always the lowest in the series, and 

 the violet at the upper end. An image of coloured light 

 so formed, is termed a spectrum (d) ; and each of the 

 rays varies, both in respect to its luminous and calorific 

 effect, as we shall perceive in our further experiments. 



Wo have stated, that seven different coloured rays may 

 be thus produced ; but, by careful examination, it will 

 be observed, that four of the colours are produced by the 

 overlapping of the other three. Hence, perhaps, in 

 strict truth, we only discover three primary colours in a 

 ray of light; namely, blue, yellow, and red. Thus, 

 green is at once formed by combining yellow and blue ; 

 the orange ray, by uniting red and yellow ; the violet is 

 a combination of red and blue ; and the indigo is pro- 

 duced by an intensification of the blue rays. 



In explaining the phenomena of the prismatic spec- 

 trum, we can happily call in the doctrine of undulations 

 to our aid ; and we find that the extreme violet rays are 



VOL. I. 



produced by a greater number of undulations taking 

 place at that part of the spectrum, than at the place 

 where the red rays are evident. The subjoined table 

 gives the length of these undulations for each ray of the 

 spectrum. 



Colour of Rayi. 



Extreme Ked 



Red 



Intermediate 



Orange 



Inunneduite 



Yellow 



Intermediate 



Green 



Intermediate 



Blue 



Intermediate 



Indigo 



Intermediate 



Violet 



Extreme Violet .. 



Length of the 

 Wares in decimal 

 parts of an Inch. 



O.OiKiM2GG 

 0.0000256 

 0.0000246 

 0.0000240 

 0.00002M 

 0.0000227 



. 



II IIIIIMI211 



0.0000189 

 0.00001U 

 0.00001(1 



Number of 



Undulations in 



an Inch. 



37640 

 39130 

 40720 

 41610 

 42510 

 44000 

 45600 

 47460 

 49320 

 51110 

 52910 

 54070 

 55240 

 57490 

 59750 



Number of 



Undulations 



per Second it 



Billions. 



458 

 477 

 495 

 506 

 517 

 535 

 55} 

 577 

 600 

 622 

 644 

 658 

 673 

 699 

 727 



It will at once be perceived, that a great difference 

 exists in the number of undulations which produce the 

 different coloured rays of the spectrum ; and we should 

 hero remark, that the luminous rays in which certain 

 chemical phenomena of light are produced, are those of 

 the violet end ; whilst those in which the heating effects 

 are most observed, are placed in the red terminal of the 

 spectrum. This is a point of great importance, as we 

 shall observe when we have to enter into the examina- 

 tion of the chemical effects of light, under the head of 

 Photography. 



If any coloured substance be placed in any of the pris- 

 matic rays, it will immediately assume the colour of that 

 ray ; and hence we perceive, that the cause of the colour 

 of bodies is due to the absorption of all the rays of 

 the spectrum but that which they reflect from their 

 surface. 



Whilst by means of a prism we can decompose white 

 light, and produce its colours, an experiment may be 

 tried by which the colours can be reunited, and white 

 light at once be reproduced. For this purpose, another 

 prism is to be placed on the one by which light has been 

 decomposed, so that the two may form together a square. 

 The ray of light will pass through without suffering 

 much change. Indeed, the two prisms thus become 

 equal to a solid piece of glass. A slight amount of 

 colour may be visible ; and this is owing to the refraction 

 of light occurring when it passes through a dense from a 

 rarer medium a phenomenon to which we shall have to 

 refer at length in our future pages. 



Another mode of illustrating the recomposition of 

 wliite light, is by painting on a circular card the different 



Prismatic colour^., in divisions extending from its centre, 

 f the card be made to rotate rapidly on its axis, a kind 

 of greyish-white colour is produced. Tliis experiment is 

 often employed at the lecture-table, for the purpose of 

 illustrating the composition of wliite light. 



We have already spoken of the speed with which the 

 rays of light traverse space; the experiment, however, 

 which we have just mentioned, is another illustration of 

 the rapidity with which light travels. The first experi- 

 ments which were made with respect to the speed of 

 iglit, were conducted by means of a wheel filled with 

 spokes, and which was made to rotate rapidly on its 



o 



