314 



OPTICS. 



of the image. The image, by reflection from a con- 

 cave mirror, is always before it when the object is 

 farther distant than the centre of curvature ; but 

 when it is nearer the image, is behind the mirror. 

 Let A C, fig. 12, be a concave mirror, of which the 

 o'ntre of curvature is E. Let rays fall upon it from 

 the arrow, x z, which is nearer to the mirror than 

 the centre E. The image will appear as if at x y z, 

 behind the mirror, erect, curved, and magnified. A 

 convex mirror has the same effect upon the rays of 

 light as a concave lens ; it causes them to diverge. 

 Let C D, fig. 13, be a convex mirror, and let rays 

 fall upon it from an arrow, A P, the rays A a, and 

 P b, fall upon it converging, and it will be reflected 

 convergingly in the direction, a E, b E, and the image 

 will appear as if situated at S, behind the mirror. 



Colours. The origin of colours is owing to the 

 composition which takes place in the rays of light, 

 each heterogeneous ray consisting of innumerable 

 rays of different colours: this is evident from the 

 separation that ensues in the well known experiment 

 of the prism. That branch of optics which treats of 

 the colours of light, of their physical properties, and 

 of the laws according to which light is decomposed, 

 and recomposed from its elements, is called chroma- 

 tics, from a Greek word signifying colour. A ray 

 being let into a darkened room, through a small 

 aperture, and falling on a triangular glass prism, is, 

 by the refraction of the prism, considerably dilated, 

 and will exhibit, on the opposite wall, an oblong 

 image, called a spectrum, variously coloured, the 

 extremities of which are bounded by semicircles, and 

 the sides are rectilinear. The colours are seven in 

 number, which, however, have various shades, gra- 

 dually intermixing at their juncture. Their order, 

 beginning from the side of the refracting angle of 

 the prism, is red, orange, yellow, green, blue, purple, 

 violet. The obvious conclusion from this experiment 

 is, that the several component parts of solar light have 

 different degrees of refrangibility, and that each sub- 

 sequent ray, in the order above mentioned, is more 

 refrangible than the preceding. Their different de- 

 grees of refrangibility may be proved by admitting 

 rays of red, orange, yellow, green, blue, indigo, and 

 violet light, through a small aperture, into a dark- 

 ened room, prepared as in the experiment for show- 

 ing the refractive power of water, alcohol, &c., above 

 described. We shall find that each colour has a dif- 

 ferent refractive power of its own, that of the red 

 being the least, and that of the violet the greatest. 

 The following table exhibits the result of such an 

 experiment with water : 



Red, . ... 1-3310 



Orange, . . . , . 1'3317 



Yellow l-S33fl 



Jreen, ..... 1*3358 



Blue, 1-3378 



Indigo, 1-3413 



Violet, ..... 1-3442 



Either of these rays, on being made to traverse 

 another prism, remains unalterable ; they are, there- 

 fore, all regarded as permanent, and each one distinct 

 from the other. This opinion is heightened by the 

 fact, that they undergo no manner of change by re- 

 flection ; for if any coloured body be placed in sim- 

 plified, homogeneous light, it will always appear of 

 the same colour as the light in which it is placed. 

 White is compounded of all the primary colours, 

 mixed in their due proportion ; for if a solar ray be 

 separated, by the prism, into its components, and, at 

 a proper distance, a lens be so placed as to collect 

 the diverging rays again into a focus, a paper, placed 

 perpendicularly to the rays in this point, will exhibit 

 whiteness. The same conclusion may be drawn from 

 mixing together paints of the same colour as the 

 parts of the spectrum, and in the same proportion ; 



the mixture will be white, though not of a resplen- 

 dent whiteness, because the colours mixed are less 

 bright than the primary ones : this may likewise be 

 proved by fixing pieces of cloth, of all the seven 

 different colours, on the rim of a wheel, and whirling it 

 round with great velocity ; it will appear to be white. 

 Though seven different colours are distinguishable in 

 the prismatic spectrum, yet, upon a closer examina- 

 tion, we shall see that there are, in fact, only three 

 original colours red, blue, yellow ; for the orange, 

 being situated between the red and yellow, is only 

 the mixture of these two; the green, in like manner, 

 arises from the blue and yellow ; and the violet troni 

 the blue and red. As the colour of a body, there- 

 fore, proceeds from a certain combination of the 

 primary rays which it reflects, the combination of 

 rays flowing from any point of an object will, when 

 collected by a glass, exhibit the same compound 

 colour in the corresponding point of the image. 

 Hence appears the reason why the images, formed 

 by glasses, have the colours of the object which they 

 represent. When a prism of solid glass is employed 

 for separating the rays of light, the lengths of the 

 colours are expressed as follows : red, forty-five ; 

 orange, twenty-seven ; yellow, forty ; green, sixty ; 

 blue, sixty ; indigo, forty-eight ; violet, eighty , or 

 360 in all. But these spaces vary with prisms of 

 different substances, and as they are not separated 

 by distinct limits, but shade gradually into one an 

 other, it is almost impossible to obtain any thing like 

 an accurate measure of their relative extents. 



The experiment with the prism will be more clearly 

 comprehended by a reference to the diagram, fig. 14. 

 Let a pencil of rays from the sun S enter a darkened 

 room through a hole in the window ; shut x y, the 

 natural course is along the line d ; but a glass prism 

 a c is interposed, which will refract the whole 

 pencil upwards, and it will be dispersed in diver- 

 gent lines, forming the spectrum M n on a white 

 wall behind the prism. From the different refran- 

 gibility of the rays of light, the red rays, which are 

 least refrangible, will occupy one portion of the spec- 

 trum ; and the violet, which are the most refrangi- 

 ble, another ; or the other colours will occupy their 

 respective places between these, according to their 

 degree of refrangibility. These are the colours of 

 the rainbow, and when we come to speak of natural 

 phenomena, we shall show how it is produced. 



Vision. Objects presented to the eye have their 

 images painted on the back part of the retina, the 

 rays of the incident pencils converging to their pro- 

 per foci there, by the refraction of the different 

 humours, for which purpose they are admirably 

 adapted ; for, as the distance between the back and 

 front of the eye is very small, and the rays of each 

 of the pencils that form the image fall parallel, or 

 else diverging, on the eye, a strong refractive power 

 is necessary for bringing them to their foci at the 

 retina ; but eacli of the humours, by its peculiar 

 form and density, contributes to cause a convergence 

 of the rays ; the aqueous, from its convex form ; the 

 crystalline, by its double convexity and greater den- 

 sity than the aqueous : and the vitreous, by a less 

 density than the crystalline, joined to its concave 

 form. The structure of the eyes is, in general, 

 adapted to the reception of parallel rays ; but, as the 

 distances of visible objects are various, so the eye 

 has powers of accommodating itself to rays proceed- 

 ing from different distances, by altering the distances 

 of the crystalline from the retina, which is done by 

 the action of the ciliary ligaments. Defective sight 

 arises from an incapacity of altering the position of 

 the crystalline within the usual limits: 1. when it 

 cannot be brought close enough to the cornea, near 

 objects appear indistinct; to this defect people in 



