1438 



VISION. 



absorbs almost all colours except reel, which 

 it reflects ; but those substances which reflect 

 all the rays appear white; those which ab- 

 sorb all are black. The brilliancy of tints 

 is greatly increased when viewed in light of 

 their own colour, as may be proved by throw- 

 ing the red rays from a prism upon any scarlet 

 object, or the green rays upon a green leaf. 

 The colour of transparent substances depends 

 upon their property of absorbing some of the 

 colours of white light, and transmitting others. 

 The blue tint of the atmosphere in reflected 

 light, and its red morning and evening tinge, 

 are to be ascribed to this cause. 



Dyes and paints are substances which, 

 when applied to bodies, so change their sur- 

 faces, that when seen in white light, they re- 

 flect only the particular colour of the dye or 

 paint. There are several modes by which 

 white light can be reproduced, of which a 

 simple one is, the rapid rotation of a disk 

 painted in stripes, with the prismatic colours 

 in the correct proportions. In this case, the 

 eye receives the impression at the same time, 

 and in the same place, of a red circle, an 

 orange, a yellow circle, and so on, and conse- 

 quently a white circle, since the sensation of 

 white is but the simultaneous sensation of all 

 these colours. 



As in the production of white light, it is 

 necessary that all the simple colours should 

 exist in their due proportions ; so it is evident 

 that by suppressing or increasing one the 

 harmony will be destroyed, and the light will 

 be no longer white. Thus, for instance, by 

 suppressing red, we obtain a blueish green, 

 which, compounded with red, would form 

 white light. Whenever two colours, simple 

 or compound, fulfil this condition, they are 

 said to be complementary one to the other. 

 They are as follows : 



Colour. 



Red - 

 Orange 

 Yellow 

 Green - 

 Blue - 

 Indigo - 

 Violet - 

 Black - 

 White - 



Complementary. 



- Blueish-green. 



- Blue. 



- Indigo. 



- Violet-reddish. 

 Orange-red. 



- Orange-yellow. 



- Yellow-green. 



- White. 



- Black. 



With respect to the production of light, 

 bodies are divided into luminous and non- 

 luminous ; among natural bodies some pos- 

 sess in themselves the property of exciting 

 in our eyes the sensation of brightness, or 

 light, as the sun and other heavenly bodies 

 which shine by their own light. There is 

 also chemical li^ht, or that produced by com- 

 bustion, electric light, phosphorescent light, et 

 cetera. Non- luminous bodies are such as 

 become visible only when light falls upon 

 them from some luminous source. 



Bodies are also divided into transparent 

 and opaque, in reference to their capacity for 

 transmitting light through their substance, 

 though this property depends, not merely on 



their absolute transparency, but also on the 

 density of the medium through which the 

 light passes. There is no perfectly opaque 

 or perfectly transparent substance known. 

 Diamond is nothing more than charcoal in a 

 different state of molecular aggregation, and 

 gold can be made pervious to light. On the 

 other hand, the purest air or clearest water 

 gradually extinguishes by absorption the rays 

 transmitted through them. According to 

 Bouguer the purest sea water loses all its 

 transparency at a depth of 730 feet, and the 

 reason that more stars are visible from the 

 summit of a lofty mountain than from the 

 level of the sea is, because the light from the 

 more distant stars becomes so much enfeebled 

 during its passage through the lower strata of 

 the atmosphere, that it has not sufficient 

 power to affect the sight. 



If a pencil of rays diverging from a lumi- 

 nous point fall upon the surface of a convex 

 lens, they will not all be equally refracted. 

 The ray which passes through the axis of the 

 lens will not be changed in its course, but the 

 remainder of the rays will be more and more 

 refracted in proportion as they recede from 

 the optical centre of the lens. When the 

 rays pass out from a bi-convex lens into air 

 they are refracted from a line perpendicular 

 to the point of emergence : the effect is to 

 cause them all to converge towards the cen- 

 tral ray to a point at which they meet, called 

 the focus. The distance between the focus 

 and the refracting surface is the focal dis- 

 tance or focal length, and is influenced by the 

 refracting power of the lens, the amount of 

 its curvature, and the distance of the lumi- 

 nous body. 



Parallel rays entering any plano-convex or 

 double convex lens at an equal distance from 

 its axis, are concentrated to the same focal 

 point, but as the peripheral rays are more 

 refracted than the central rays, they are sooner 

 brought to a focus ; hence the image formed 

 at the focus of the lens is somewhat indistinct 

 at its edges. This imperfection is due to 

 what is termed spherical aberration, and is 

 counteracted either by shutting out the peri- 

 pheral rays, or by such a combination of lenses 

 as will establish a just proportion between the 

 refraction of the central and peripheral rays. 

 Such lenses are made of crown glass, com- 

 posed of flint and alkali only, and flint glass, 

 in which oxide of lead is added to the other 

 materials. The latter possesses a much higher 

 dispersive power than the crown glass ; but 

 the refraction of the rays is nearly the same 

 in both, and when combined, achromatic lenses 

 are obtained. This term is applied from their 

 utility in obviating another source of con- 

 fusion, chromatic aberration, which is caused 

 by the unequal refrangibility of the prismatic 

 rays when transmitted through an ordinary 

 lens, whereby the images are fringed with 

 colours, and are rendered even more indis- 

 tinct than by spherical aberration. Newton 

 supposed that an achromatic lens was an im- 

 possibility ; but in 1757 Dollond completely 

 succeeded in overcoming the difficulty by the 





