DIFPKACTIOS. ] 



UNDULATORY FORCES. LIGHT. 



needle at any place beyond the focus whence the rays 

 diverge. If the shadow of the needle be received on a 

 sheet of paper, fringes of coloured light will be observed 

 thereon. A convenient instrument for the purpose may 

 easily be constructed out of a common camera, by re- 

 moving the ground-glass plate and the mirror, and by 

 replacing the convex lens in front by one of a very short 

 focus say not exceeding an inch. The opposite end is 

 covered with a sheet of white paper ; and the object 

 whose shadow is to be obtained, must be placed between 

 the paper and the focal point of the rays passing from 

 the sun and through the lens. The observer can thus 

 perceive the coloured fringes on the paper by looking 

 through the open door generally found in the old form 

 of the camera. Our annexed illustration will give an 

 idea of the construction of this apparatus. 

 Fi. 3*. 



mm mm, itfrtmmt the unal form of the camera-box ; e, * lew of ihort 

 (MM; , aeedle producinj the shadow ; m m, the coloured fringn 

 meirtd on a paper eerae*. rwUnf on the end of the boi ; t, a rajr of 

 llrtl entcrinf the lew ; /, poaiUoas of the rye required to obterre the 

 nniri by reflection. 



The above arrangement, whilst extremely simple, has 

 the advantage of being easily held in such a position as 

 to be at any time in the direct rays of the sun, or of an 

 artificial light. The end of the box, a d d a, may be 

 taken out and replaced by a sheet of ground glass, when 

 the fringes may be observed through it, as at g, instead 

 of on the surface of the paper screen. The observer will 

 perceive that the white light has been completely decom- 

 posed, and the fringes present the same colours as if pro- 

 duced by the glass prism. The first set of fringes exhibits 

 the colours violet, blue, green, yellow, and red ; the next, 

 blue, yellow, and red; and the third, the same colours 

 a* the second, but fainter in their tint. If the object by 

 which the diverging rays pass be moved to and fro be- 

 tween the light and the screen receiving the fringes, the 

 latter increase in width. 



The effects thus observed are to be explained on iden- 

 tical principles with those which we have already referred 

 to when describing the phenomena of the interference of 

 two sets of undulations. And the fringes of colour, or 

 the dark shadows, are simply due to undulations of 

 different lengths interfering with each other. Having 

 already entered fully into the philosophy of the ques- 

 tion,* we pas* on to consider some modification of these 

 experiments. 



Experinviit 3. In the diverging beam place a piece of 

 metal through which a very fine hole has been made by 

 of a pin's point, and between this and the eye 

 a convex lens. A circle of beautifully coloured 

 rings will be at once observed, which will expand or con- 

 tract as the lens is shifted to and fro between the eye 

 and the hole in the metal plate. A piece of thick tin- 

 foil answers as well for this experiment as the metal plate, 

 if a hole be pierced through it, as already described. 



A great variety of arrangements, wherein two surfaces 

 placed close together and introduced into the diverging 

 beam, may be employed. Amongst these may be two 

 knives, whose edges are together at one end, and slightly 

 separated at the other ; gauze-work, either of vegetable 

 ti' TO or of metal wires ; perforations in any opaque object ; 

 and, iii'lued, any objuct which by interfering with the 

 continuity of the ray* of the diverging beam, will tend 

 to produce the phenomena we have mentioned. 



In certain instances, the centre of the shallow will pre- 



'inlf, p. SI. 



sent a black line, or point, where the undulations have 

 destroyed each other by their mutual interference. In 

 other cases a line of white light is produced. The student 

 will here perceive a practical illustration of our previous 

 remarks on the production of light, or its absence, by the 

 motion of the waves of ether being destroyed in certain 

 cases, and intensified in others, just as these undulations 

 do or do not interfere with each other. 



It will be noticed that the rays passing from an inter- 

 fering object are not in straight lines. They follow the 

 curve of a hyperbola, and so reach the screen, as men- 

 tioned in our description of the simple apparatus whose 

 use we suggested when introducing this subject. 



DISPERSION AND ACHROMATISM. 



WK have already mentioned, that different transparent 

 media have varied powers in dispersing or spreading the 

 rays of light transmitted through them. \Ve now pro- 

 pose to investigate the subject more fully, and also to 

 refer to a very valuable application of the laws we shall 

 meet with; namely, the construction of achromatic 

 lenses. 



If solar light be transmitted through two lenses, one of 

 crown and the other of flint glass, being similar in every 

 respect excepting in the material of which they are ni.udi', 

 it will be found that the spectra they produce differ very 

 materially. They affect tliu h-n-th and position of the 

 spectrum produced that of llint jrlass acting to the 

 greatest extent in these respects. Tliis is owing to the 

 fact, that flint glass has a greater dispersive power on the 

 rays of light than crown glass. Moreover, it is found, 

 that certain substances extend one end of the spectrum 

 more than others, some having a special influence on the 

 blue, and others on the red rays ; and thus either of 

 these predominate over the other, according to the mate- 

 rial of which the prism is composed ; hence arises that 

 which has been called the irrationality of the spectrum. 



As we hove already remarked, two prisms of different 

 material may be so arranged, that the light passing 

 through them shall not be separated into a spectrum, 

 and but comparatively little coloured light will thereby 

 be produced. By an analogical process, achromatic 

 lease* are constructed ; that is, lenses which do not give 

 coloured images such as those which one material always 

 flbtdi 



To understand what is called "chromatic aberration," 

 or the cause of these coloured images when ordinary 

 lenses are employed, the student may receive the rays of 

 the sun through a lens on a sheet of white paper. If 

 the light reach the paper at a less distance than that at 

 which the general focal point exists from the lens, then 

 he will perceive a ring of a blue colour. If the paper bo 

 placed further from the lens than the focal point, then 

 a red ring will be observed. This effect may often be 

 noticed when the lenses of a common magic lantern are 

 being focussed, the light proceeding therefrom being re- 

 ceived on a white screen. A red, or blue hazy colour is 

 thus produced, and is an instance illustrating the fact, 

 that the red and blue rays focus at different points after 

 proceeding from the lens, owing to unequal refrangibility. 

 This effect is what is called chromatic aberration. 



The student will observe the general effect of chro- 

 matic aberration by looking at an object through a com- 

 mon lens, when he will perceive the external parts of the 

 image to be coated with coloured fringes. 



This gives rise to great inconvenience when such lenses 

 are employed in telescopes, microscopes, photographic 

 cameras, and other optical instruments. However, a 

 knowledge of the cause of any defect very often suggests 

 its remedy ; and this occurs in reference to the subject 

 with which we are now dealing. 



It lias been found that, by combining lenses composed 

 of materials having different dispersive powers, a single 

 lens almost achromatic that is, giving no coloured 

 images may be produced which affords a focus where all 

 the rays meet simultaneously. In practice, this is done 

 by combining a flint with a crown or plate glass lens. 

 In some instances more than two lenses are employed ; 



