50 



UNDULATORY FORCEa LIGHT. 



[THK RAINUOW. 



how much more heat is to bo found in the red than in 

 the blue rays ; and if a piece of note-paper be well soaVed 

 in a solution of common salt, and, when dry, moistened 

 with a solution of nitrate of silver, it will be a tost of the 

 power of the chemical rays. If placed in the blue or 

 violet band, it will be speedily darkened, owing to the 

 chemical action of the violet ray on the salt of silver on 

 its surface. In the red ray it will scarcely undergo any 

 perceptible change. 



The most convenient mode of conducting this instruc- 

 tive experiment, is by stretching a piece of paper, pre- 

 pared as above, along the entire length of the spectrum, 

 so that all the rays from the red to the extreme violet 

 may fall on it. The paper will be darkened just in pro- 

 portion to the energy of the blue rays, and unaffected in 

 the yellow and rod. Both the calorific and chemical rays 

 extend, however, beyond the visible spectrum, and are 

 more energetic than at the point where light ceases to be 

 perceived : of this, however, we shall speak fully here- 

 after. 



The well-known phenomenon of the rainbow is easily 

 explained, on principles similar to those involved in tlie 

 production of the prismatic spectrum. Indeed, each 

 drop of rain, by refracting the rays of light incident on 

 it, acts as a prism, and produces the coloured image 

 which is impressed on the retina of the eye. The reason 

 of this will be better understood if we examine the effect 

 produced on a ray of sunlight falling on the rain-drops, 

 as illustrated in the annexed engraving. (Fig. 24.) 



Fig. 24. ^a " e w iU SU P~ 



pose a to be a 

 ray of light fall- 

 ing on the rain- 

 drop b, the sun 

 being behind 

 the spectator, 

 and the rain- 

 drop facing 

 him, and at a 

 certain angle 

 with the sun 

 and his eye. 

 On entering the drop, the ray is, of course, immediately 

 refracted, passing as it does, in an oblique direction, 

 from a rare into a dense medium. It will arrive at 

 some point in the drop (say c), and a portion of it will 

 be transmitted. Another part, however, will be re- 

 flected to d ; and as it leaves the drop the emerging ray 

 will be divided, and its colours will be evident to the 

 spectator at e. It will be at once perceived, therefore, 

 that the rain-drop acts precisely as a prism ; and in like 

 manner the different coloured rays reach the eye, in the 

 same order as in the prismatic spectrum ; the blue rays 

 being observed at the top, and the red rays at the lower 

 part of the rainbow. Of course, what is true of one 

 drop is equally applicable to all, and their great number 

 combines to produce the unparalleled splendour which 

 the "bow in the heavens" presents to the eye of the 

 admiring observer. With respect to the order in which 

 the colours of the rainbow are seen, we must add, that 

 each coloured ray has a special angle of its own ; and as 

 these emerge from each drop of water, they successively 

 reach the eye in the order of the least refrangible or red 

 rays in an inferior, and the more refrangible or blue 

 rays in the superior, position. The student will thus 

 perceive a beautiful illustration of all the laws of reflec- 

 tion and refraction to which wo have already alluded in 

 the phenomena of the bow, whether produced by the 

 shower of rain, or by any means where water, in a minute 

 state of division, can be placed in the proper angle of 

 light incident on it. 



A most beautiful effect is produced by casting the 

 prismatic spectrum on steam rising from hot water, 

 which affords an appearance much resembling a rainbow. 

 Artificial means can easily be employed for producing 

 the appearances of a "bow," by casting light on water 

 issuing under pressure from an orifice, so as to form 

 pray. In a similar manner a "rainbow" may often be 

 observed over a waterfall, and at the bows of a steamer, 



Fig. K. 



ft 



or near the paddle-wheels when they are in motion, be- 

 cause the water thus cast up is in a favourable state of 

 minute division for affording the bow-like appearance on 

 a small scale. 



The rich variety of colours observed during sunrixn 

 and sunset, is equally due to the refraction of light 

 produced by the moisture held in the state of vapour in 

 the atmosphere. Generally speaking, the more <! 

 the state of the vapour or clouds, the more inclined to a 

 red appearance is the effect produced ; and by a gradation 

 in the amount of moisture present, every variety of tint 

 is thus produced. Hence, we invariably find, that the 

 colour of the clouds towards sunset is of a deep red 

 tint, because the refraction is greater at the horizon than 

 at the zenith, whether in the presence or absence of 

 cloud. There are many other appearances of coloured 

 light observed round the sun and moon during cloudy 

 weather ; and all these are easily explained, on the prin- 

 ciple of the refraction of light, by its passage through, 

 or reflection from, globules of aqueous vapour. 



If the student place a round and solid ball of glass 

 so that the light of a caudle may impinge on it in a 

 manner similar to that observed when the sun produces 

 the rainbow, he may easily observe all the effects of 

 coloured light, produced by the rays from the candle 

 being refracted by the glass. 



The best mode of 

 carrying out this sim- 

 ple experiment, is that 

 represented in Fig. 25, 

 where o represents the 

 flame of a candle, b a 

 solid globe of glass an 

 inch in diameter, and 

 c the position of the 

 eye, some way between 

 the glass and the can- 

 dle, so that the latter may be behind the spectator. The 

 ray of light will pass to the back of the glass globe, and, 

 having been refracted, the observer, if placed at angles 

 varying from 40 to 50 with a straight line drawn from 

 the glass globe, between the eye and the candle, will 

 perceive the coloured rays of light in their natural order 

 of succession. As the head is moved towards the light, 

 the blue rays will be perceived as reflected from the 

 glass ; but, on gently moving the eye from the candle, 

 all the prismatic colours will gradually present themselves. 

 An easily understood experiment will be thus afforded, 

 giving a good insight into the laws which govern the 

 production of coloured light in atmospheric phenomena. 

 Even a glance at an ordinary chandelier pendant, will 

 give the same effects, and is a still more common illus- 

 tration of the points we have mentioned. 



On similar causes to those we have here explained, 

 the beautiful play of colours seen when a brilliant is 

 viewed by reflected light, depend. The light is refracted 

 by entering the substance of the diamond, and, on sub- 

 sequent reflection, the coloured rays are perceived by 

 the eye. 



The colour of all bodies is simply caused by their 

 absorbing every ray of the prismatic spectrum, except 

 that which reaches the eye. Thus, if we look at a red 

 article, we state, as the reason of its colour, that all the 

 rays of every other coloured light are absorbed by the 

 material, that of red only being reflected from its sur- 

 face. Of course, the same argument is applied in refer- 

 ence to all colours which can be perceived. We must 

 here refer to those admirable researches instituted by 

 Sir Isaac Newton, which have thrown great light on this 

 most interesting subject 



Every one must have noticed, that when oil, tar, and 

 other liquids are dropped on the surface of water, a 

 variety of beautiful colours is produced. Sir Isaac 

 Newton, by forming thin plates of air through pressing 

 two glass surfaces together, was enabled to even measure 

 the thickness of those plates for each colour afforded. 

 As these results are of great importance, we give direc- 

 tions, by following which the student may easily repro- 

 duce them at his convenience. 



