CHAPTER VI.

 Colours of thin Plates first studied by Boyle and Hooke—Newton determines the Law of their Production—His Theory of Fits of Easy Reflection and Transmission—Colours of thick Plates.

In examining the nature and origin of colours as the component¹ parts of white light, the attention of Newton was directed to the curious subject of the colours of thin plates, and to its application to explain the colours of natural bodies. His earliest researches on this subject were communicated, in his Discourse² on Light and Colours, to the Royal Society, on the 9th December, 1675, and were read at subsequent meetings of that body. This discourse contained fuller details respecting the composition and decomposition³ of light than he had given in his letter to Oldenburg, and was concluded with nine propositions, showing how the colours of thin transparent⁴ plates stand related to those of all natural bodies.

The colours of thin plates seem to have been first observed by Mr. Boyle. Dr. Hooke afterward⁵ studied them with some care, and gave a correct account of the leading phenomena⁶, as exhibited in the coloured rings upon soap-bubbles, and between plates of glass pressed together. He recognised that the colour depended upon some certain thickness of the transparent plate, but he acknowledges that he had attempted in vain to discover the relation between the thickness of the plate and the colour which it produced.

Dr. Hooke succeeded in splitting a mineral substance, called mica⁷, into films of such extreme thinness as to give brilliant colours. One plate, for example,76 gave a yellow colour, another a blue colour, and the two together a deep purple; but, as plates which produced those colours were always less than the 12,000th part of an inch thick, it was quite impracticable, by any contrivance yet discovered, to measure their thickness, and determine the law according to which the colour varied⁸ with the thickness of the film. Newton surmounted⁹ this difficulty by laying a double convex lens, the radius¹⁰ of curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in this way he obtained a plate of air or of space varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface, to a considerable thickness at the circumference¹¹ of the lens. When light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glass gave different colours, so that the point where the two object-glasses touched one another was the centre of a number of concentric coloured rings. Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena.

In order to understand how he proceeded, let CED be the convex surface of the one object-glass, and AEB the flat surface of the other. Let them touch at the point E, and let homogeneous red rays fall upon them, as shown in the figure. At the point of contact E, where the plate of air is inconceivably thin, not a single ray of the pencil RE is reflected. The light is wholly transmitted, and, consequently, to an eye above E, there will appear at E a black spot. At a, where the plate of air is thicker, the red light ra is reflected in the direction aa′, and as the air has the same thickness in a circle round the point E, the eye above E, at a, will see next the black spot E a ring of red light. At m,77 where the thickness of the air is a little greater than at a, the light r′m is all transmitted as at E, and not a single ray suffers reflection, so that to an eye above E at m′ there will be seen without the red ring a a dark ring m. In like manner, at greater thicknesses of the plate of air, there is a succession of red and dark rings, diminishing in breadth as shown in the diagram.



When the same experiment was repeated in orange, yellow, green, blue, indigo¹³, and violet light, the very same phenomenon was observed; with this difference only, that the rings were largest in red light, and smallest in violet light, and had intermediate magnitudes in the intermediate colours.

If the observer now places his eye below E, so as to see the transmitted rays, he will observe a set of rings as before, but they will have a bright spot in their centre at E, and the luminous¹⁴ rings will now correspond with those which were dark when seen by reflection, as will be readily understood from inspecting the preceding diagram.

When the object-glasses are illuminated¹⁵ by white78 light, the seven systems of rings, formed by all the seven colours which compose white light, will now be seen at once. Had the rings in each colour been all of the same diameter they would all have formed brilliant white rings, separated by dark intervals¹⁶; but, as they have all different diameters, they will overlap¹⁷ one another, producing rings of various colours by their mixture. These colours, reckoning from the centre E, are as follows:—

1st Order. Black, blue, white, yellow, orange, red.

2d Order. Violet, blue, green, yellow, orange, red.

3d Order. Purple, blue, green, yellow, red, bluish-red.

4th Order. Bluish-green, green, yellowish-green, red.

5th Order. Greenish-blue, red.

6th Order. Greenish-blue, red.

By accurate measurements, Sir Isaac found that the thicknesses of air at which the most luminous parts of the first rings were produced, were in parts of an inch 1/178000, 3/178000, 5/178000, 7/178000, 9/178000, 11/178000. If the medium or the substance of the thin plate is water, as in the case of the soap-bubble, which produces beautiful colours according to its different degrees of thinness, the thicknesses at which the most luminous parts of the rings appear are produced at 1/1·336 of the thickness at which they are produced in air, and in the case of glass or mica at 1/1·525 of that thickness; the numbers 1.336, 1.525 expressing the ratio of the sines of the angles of incidence and refraction in the substances which produce the colours.

From the phenomena thus briefly¹⁸ described, Sir Isaac Newton deduces that ingenious, though hypothetical, property of light, called its fits of easy reflection and transmission. This property consists in supposing that every particle of light from its first discharge from a luminous body possesses, at equally distant intervals, dispositions²⁰ to be reflected from,79 and transmitted through, the surfaces of bodies upon which it is incident. Hence, if a particle of light reaches a reflecting surface of glass when it is in its fit of reflection, or in its disposition¹⁹ to be reflected, it will yield more readily to the reflecting force of the surface; and, on the contrary, if it reaches the same surface while in a fit of easy transmission, or in a disposition to be transmitted, it will yield with more difficulty to the reflecting force. Sir Isaac has not ventured to inquire into the cause of this property; but we may form a very intelligible²¹ idea of it by supposing, that the particles of light have two attractive and two repulsive²² poles at the extremities²³ of two axes at right angles to each other, and that the particles revolve²⁴ round their axes, and at equidistant intervals bring one or other of these axes into the line of the direction in which the particle is moving. If the attractive axis²⁵ is in the line of the direction in which the particle moves when it reaches the refracting surface, the particle will yield to the attractive force of the medium, and be refracted and transmitted; but if the repulsive axis is in the direction of the particle’s motion when it reaches the surface, it will yield to the repulsive force of the medium, and be reflected from it.

The application of the theory of alternate fits of reflection and transmission to explain the colours of thin plates is very simple. When the light falls upon the first surface AB, Fig¹². 8 of the plate of air between AB and CED, the rays that are in a fit of reflection are reflected, and those that are in a fit of transmission are transmitted. Let us call F the length of a fit, or the distance through which the particle of light moves while it passes from the state of being in a fit of reflection to the state of being in a fit of transmission. Now, as all the particles of light transmitted through AB were in a state of easy transmission when they entered AB, it is obvious, that, if the plate of air at E is so thin as to be less80 than one-half of F, the particles of light will still be in their disposition to be transmitted, and consequently the light will be all transmitted, and none reflected at the curve surface at E. When the plate becomes thicker towards a, so that its thickness exceeds half of F, the light will not reach the surface CE till it has come under its fit of reflection, and consequently at a the light will be all reflected, and none transmitted. As the thickness increases towards m, the light will have come under its fit of transmission, and so on, the light being reflected at a, l, and transmitted at E, m. This will perhaps be still more easily understood from fig. 9, where we may suppose AEC to be a thin wedge of glass or any other transparent body. When light is incident on the first surface AE, all the particles of it that are in a fit of easy reflection will be reflected, and all those in a fit of easy transmission will be transmitted. As the fits of transmission all commence at AE, let the first fit of transmission end when the particles of light have reached ab, and the second when they have reached ef; and let the fits of reflection commence at cd and gh. Then, as the fit of transmission continues from AE to ab, all the light that falls upon the portion mE of the second surface will be transmitted and none reflected, so that to an eye above E the space mE will appear black. As the fit of reflection commences at ab, and81 continues to cd, all the light which falls upon the portion nm will be reflected, and none transmitted; and so on, the light being transmitted at mE and pn, and reflected at nm and qp. Hence to an eye above E the wedge-shaped film of which AEC is a section will be covered with parallel bands or fringes of light separated by dark fringes of the same breadth, and they will be all parallel to the thin edge of the plate, a dark fringe corresponding to the thinnest edge. To an eye placed below CE, similar fringes will be seen, but the one corresponding to the thinnest edge mE will be luminous.



If the thickness of the plate does not vary according to a regular law as in fig. 9, but if, like a film of blown glass, it has numerous inequalities, then the alternate fringes of light and darkness will vary with the thickness of the film, and throughout the whole length of each fringe the thickness of the film will be the same.

We have supposed in the preceding illustration that the light employed is homogeneous. If it is white, then the differently coloured fringes will form by their superposition a system of fringes analogous²⁶ to those seen between two object-glasses, as already explained.

The same periodical colours which we have now described as exhibited by thin plates were discovered by Newton in thick plates, and he has explained them by means of the theory of fits; but it would lead us beyond the limits of a popular work like this to enter into any details of his observations, or to give an account of the numerous and important additions which this branch of optics has received from the discoveries of succeeding authors.

1 component   

n.组成部分，成分，元件；adj.组成的，合成的

参考例句：

• Each component is carefully checked before assembly.每个零件在装配前都经过仔细检查。
• Blade and handle are the component parts of a knife.刀身和刀柄是一把刀的组成部分。

2 discourse   

n.论文，演说；谈话；话语；vi.讲述，著述

参考例句：

• We'll discourse on the subject tonight.我们今晚要谈论这个问题。
• He fell into discourse with the customers who were drinking at the counter.他和站在柜台旁的酒客谈了起来。

3 decomposition   

n. 分解, 腐烂, 崩溃

参考例句：

• It is said that the magnetite was formed by a chemical process called thermal decomposition. 据说这枚陨星是在热分解的化学过程中形成的。
• The dehydration process leads to fairly extensive decomposition of the product. 脱水过程会导致产物相当程度的分解。

4 transparent   

adj.明显的，无疑的；透明的

参考例句：

• The water is so transparent that we can see the fishes swimming.水清澈透明，可以看到鱼儿游来游去。
• The window glass is transparent.窗玻璃是透明的。

5 afterward   

adv.后来；以后

参考例句：

• Let's go to the theatre first and eat afterward. 让我们先去看戏，然后吃饭。
• Afterward,the boy became a very famous artist.后来，这男孩成为一个很有名的艺术家。

6 phenomena   

n.现象

参考例句：

• Ade couldn't relate the phenomena with any theory he knew.艾德无法用他所知道的任何理论来解释这种现象。
• The object of these experiments was to find the connection,if any,between the two phenomena.这些实验的目的就是探索这两种现象之间的联系，如果存在着任何联系的话。

7 mica   

n.云母

参考例句：

• It could not pass through material impervious to water such as mica.它不能通过云母这样的不透水的物质。
• Because of its layered structure,mica is fissile.因为是层状结构，云母很容易分成片。

8 varied   

adj.多样的，多变化的

参考例句：

• The forms of art are many and varied.艺术的形式是多种多样的。
• The hotel has a varied programme of nightly entertainment.宾馆有各种晚间娱乐活动。

9 surmounted   

战胜( surmount的过去式和过去分词 )； 克服（困难）； 居于…之上； 在…顶上

参考例句：

• She was well aware of the difficulties that had to be surmounted. 她很清楚必须克服哪些困难。
• I think most of these obstacles can be surmounted. 我认为这些障碍大多数都是可以克服的。

10 radius   

n.半径，半径范围；有效航程，范围，界限

参考例句：

• He has visited every shop within a radius of two miles.周围两英里以内的店铺他都去过。
• We are measuring the radius of the circle.我们正在测量圆的半径。

11 circumference   

n.圆周，周长，圆周线

参考例句：

• It's a mile round the circumference of the field.运动场周长一英里。
• The diameter and the circumference of a circle correlate.圆的直径与圆周有相互关系。

12 fig   

n.无花果(树)

参考例句：

• The doctor finished the fig he had been eating and selected another.这位医生吃完了嘴里的无花果，又挑了一个。
• You can't find a person who doesn't know fig in the United States.你找不到任何一个在美国的人不知道无花果的。

13 indigo   

n.靛青，靛蓝

参考例句：

• The sky was indigo blue,and a great many stars were shining.天空一片深蓝，闪烁着点点繁星。
• He slipped into an indigo tank.他滑落到蓝靛桶中。

14 luminous   

adj.发光的，发亮的；光明的；明白易懂的；有启发的

参考例句：

• There are luminous knobs on all the doors in my house.我家所有门上都安有夜光把手。
• Most clocks and watches in this shop are in luminous paint.这家商店出售的大多数钟表都涂了发光漆。

15 illuminated   

adj.被照明的；受启迪的

参考例句：

• Floodlights illuminated the stadium. 泛光灯照亮了体育场。
• the illuminated city at night 夜幕中万家灯火的城市

16 intervals   

n.[军事]间隔( interval的名词复数 )；间隔时间；[数学]区间；（戏剧、电影或音乐会的）幕间休息

参考例句：

• The forecast said there would be sunny intervals and showers. 预报间晴，有阵雨。
• Meetings take place at fortnightly intervals. 每两周开一次会。

17 overlap   

v.重叠，与…交叠；n.重叠

参考例句：

• The overlap between the jacket and the trousers is not good.夹克和裤子重叠的部分不好看。
• Tiles overlap each other.屋瓦相互叠盖。

18 briefly   

adv.简单地，简短地

参考例句：

• I want to touch briefly on another aspect of the problem.我想简单地谈一下这个问题的另一方面。
• He was kidnapped and briefly detained by a terrorist group.他被一个恐怖组织绑架并短暂拘禁。

19 disposition   

n.性情，性格；意向，倾向；排列，部署

参考例句：

• He has made a good disposition of his property.他已对财产作了妥善处理。
• He has a cheerful disposition.他性情开朗。

20 dispositions   

安排( disposition的名词复数 )； 倾向； （财产、金钱的）处置； 气质

参考例句：

• We got out some information about the enemy's dispositions from the captured enemy officer. 我们从捕获的敌军官那里问出一些有关敌军部署的情况。
• Elasticity, solubility, inflammability are paradigm cases of dispositions in natural objects. 伸缩性、可缩性、易燃性是天然物体倾向性的范例。

21 intelligible   

adj.可理解的，明白易懂的，清楚的

参考例句：

• This report would be intelligible only to an expert in computing.只有计算机运算专家才能看懂这份报告。
• His argument was barely intelligible.他的论点不易理解。

22 repulsive   

adj.排斥的，使人反感的

参考例句：

• She found the idea deeply repulsive.她发现这个想法很恶心。
• The repulsive force within the nucleus is enormous.核子内部的斥力是巨大的。

23 extremities   

n.端点( extremity的名词复数 )；尽头；手和足；极窘迫的境地

参考例句：

• She was most noticeable, I thought, in respect of her extremities. 我觉得她那副穷极可怜的样子实在太惹人注目。 来自辞典例句
• Winters may be quite cool at the northwestern extremities. 西北边区的冬天也可能会相当凉。 来自辞典例句

24 revolve   

vi.(使)旋转；循环出现

参考例句：

• The planets revolve around the sun.行星绕着太阳运转。
• The wheels began to revolve slowly.车轮开始慢慢转动。

25 axis   

n.轴，轴线，中心线；坐标轴，基准线

参考例句：

• The earth's axis is the line between the North and South Poles.地轴是南北极之间的线。
• The axis of a circle is its diameter.圆的轴线是其直径。

26 analogous   

adj.相似的；类似的

参考例句：

• The two situations are roughly analogous.两种情況大致相似。
• The company is in a position closely analogous to that of its main rival.该公司与主要竞争对手的处境极为相似。