OPTICS. 



CHAPTER I. OPTICS. 



Definitions Nature of Light Its ge- 

 neral Properties. 



OPTICS, a term derived from a Greek 

 word, which signifies seeing, is the sci- 

 ence which treats of vision, and generally 

 of the nature and properties of Light, 

 and the changes which it undergoes in 

 its qualities or in its direction, when 

 passing through bodies of different 

 shapes ; when reflected from their sur- 

 faces or when moving past them at 

 small distances. 



The nature of Light has not yet been 

 ascertained ; but two opinions of a very 

 opposite kind have been maintained by 

 philosophers respecting its origin and 

 propagation. By some it is supposed 

 to consist of material particles, thrown 

 off from the luminous body with great 

 velocity, and in all directions, while 

 others believe it to be j a fluid dif- 

 fused through all nature, and in which 

 waves or undulations are produced by 

 the action of the luminous body, and 

 propagated in the same manner as sound 

 is propagated through air. 



But whatever be the cause of light, 

 and whatever be the manner in which 

 it passes from one point of space to 

 another, it has certain general properties 

 which have been discovered by experi- 

 ment and observation. 



1 . Light is sent forth in all directions 

 from every visible point of luminous 

 bodies. If we hold a sheet of paper be- 

 fore a candle, or the sun, or a red hot 

 body, or any other source of light, we 

 shall find that the paper is illuminated, 

 or covered with light, in whatever posi- 

 tion we hold it, provided that the light 

 is not obstructed by its edge, or by 

 another body. If we examine the illu- 

 minated surface, we shall also find that 

 there are no black spaces or intervals 

 destitute of light. 



"I. All bodies, whether natural or arti- 

 ficial, which are not luminous of them- 

 selves, are rendered visible by light which 

 originally comes from a body luminous 



of itself, such as the sun, or common ar- 

 tificial lights. When the sun shines on 

 a plant, the plant is seen in great bril- 

 liancy and beauty. If a black cloud 

 covers the sun, the plant is still seen, 

 though with less brightness, but the light 

 which now makes it visible comes from 

 the clouds, which are illuminated by the 

 sun : for when the sun is so far below 

 the horizon as not to illuminate the 

 clouds, the plant ceases to be visible. 

 In like manner, when we read by the 

 artificial light of a candle, the book 

 is generally illuminated by the light 

 which immediately proceeds from it ; 

 but we can still read with our back to 

 the candle, in which case the book is 

 illuminated by the light from the candle 

 which falls upon the walls arid furniture 

 of the apartment, and which those walls 

 and that furniture again throw off in all 

 directions ; for the instant the candle is 

 put out we are left in total darkness. 



3. All bodies, whether natural or arti- 

 ficial, throw off in all directions light of 

 the same colour as themselves, although 

 the light from the sun, which falls upon 

 them and renders them visible, is white. 

 It has been generally supposed to be a 

 sufficient proof of this important pro- 

 perty, to state, that wherever we place 

 our eye it sees those bodies of that co- 

 lour ; but as we consider this unsatis- 

 factory, we shall demonstrate it by an 

 experiment which is both beautiful and 

 instructive. If we hold a white card 

 before a rose bush, the surface of the 

 card will appear of its usual whiteness ; 

 but if we place this card at one end of a 

 box shut up on all sides, and if, having 

 made a pin hole in the side opposite the 

 card to admit the light from the rose 

 bush, we look through another hole at 

 the card, we shall see upon the card, and 

 opposite each rose, a patch of red light, 

 and opposite each green leaf a patch of 

 green light. These patches of colour 

 constitute a picture of the rose bush 

 turned upside down, which though not 

 ' very distinct in the outline, will yet be 

 easily recognised. If we enlarge the small 

 hole opposite the card, the picture will 



