774 PERCEPTION OF COLOURS. 



noon sensorv nerves. About 90 per cent, of these rays is absorbed by the media of the eye 

 rnr^7S^,H From Frauenhofer's line, A, onwards the oscillations of the light- 

 ther excite the retina in the following order .-Red with 481 billions of vibmtions per second 

 orange with 532, yeUmo with 563, green with 607, bine with 653, indigo with ^676, and violet 

 with 764 billion Vibrations per second. The sensation of colour therefore depends on the 

 number pf vibrations of the fight-ether, just as the pitch of a note depends on the number of 

 vibrations of the sounding body {Xewton, 1704 ; Hartley, 17*2). Beyond the violet lie the 

 chemically active [actinic] ravs of the spectrum. After cutting out all the spectrum, including 

 the violet rays, v. Helmholtz succeeded in seeing the ultra-violet rays, which had a feeble 

 erevish-blue colour. The heat-rays in the coloured part of the spectrum are transmitted by the 

 media of the eve in the same way as through water. The existence of the ultra-violet rays is 

 best ascertained by the phenomenon of fluorescence. Von Helmholtz, on illuminating a solution 

 of sulphate of quinine with the ultra-violet rays, saw a bluish-white light proceeding from all 

 parts of the solution which were acted on by the ultra-violet rays. As the media of the eye them- 

 selves exhibit fluorescence (v. Helmholtz), they must increase the power of the retina to distin- 

 guish these rays. The ultra-violet rays are not largely absorbed by the media of the eye (Briickc). 

 n In order that a colour be perceived, it is essential that a certain amount of light fall upon the 

 retina. Blue, when at the lowest degree of brightness, gives a colour sensation with an amount 

 of light which is sixteen times less than that required for red (Dobrowlosky). 



Intensity of the Impression of Light. While light of different periods of vibration applied 

 to the eye excites the different sensations of colour, the amplitude of the vibrations (height of 

 the waves) determines the intensity of the impression of light ; just as the loudness of a note 

 depends on the amplitude of the vibrations of the sounding body. The sun's light contains all 

 the rays which excite the sensation of colour in us, and when all these rays fall simultaneously 

 upon the retina we experience the sensation of white. If the colours of the spectrum obtained 

 by means of a prism be reunited, white light is again obtained. If no vibrations of the light- 

 ether reach the retina, every sensation of light and colour is absent, but we can scarcely apply the 

 term black to this condition. It is rather the absence of sensation, such as, for example, is 

 the case when a beam of light falls on the skin of the back. This does not give the sensation 

 of black, but rather that of no sensation of light. 



Simple and Mixed Colours. We distinguish simple colours, e.g., those of the 

 spectrum. In order to perceive these, the retina must be excited (set into vibration) 

 by a distinct number of oscillations (see above). Further, we distinguish " mixed 

 colours," whose sensations are produced when the retina is excited by two or more 

 simple colours, simultaneously or rapidly alternating. The most complex mixed 

 colour is white, which is composed of a mixture of all the simple colours of the 

 spectrum. 



The " complementary colours " are important. Any two colours which together 

 give the sensation of white are complementary to each other. The " contrast colours " 

 are mentioned here merely to complete the list. They are closely related to the 

 complementary colours. Any two colours which, when mixed, supplement the 

 generally prevailing tone of the light, are contrast colours. When the sky is blue, 

 the two contrast colours must be bluish-white : with bright gaslight they must be 

 yellowish-white, and in pure white light of course all the complementary are the 

 same as the contrast colours (Biiicke). 



Methods of Mixing Colours. 1. Two solar spectra are projected upon a screen, and the 

 spectra are so arranged as to cause anv one part of one spectrum* to cover any part of the other. 

 2. Look obliquely through a vertically arranged glass plate at a colour placed behind it. 

 Auother colour is placed in front of the glass plate, so that its image is also reflected into the 

 eye of the observer ; thus, the light of one colour transmitted through the glass plate and the 

 reflected light from the other colour reach the eye simultaneously. [Lambert's Method. This 

 is easily done by Lambert's method. Use coloured wafers and a slip of glass ; place a red wafer 

 on a sheet of black paper, and about 3 inches behind it another blue one. Hold the plate of 

 glass midway and vertically between them, and so incline the glass that, while looking.through 

 it at the red wafer, a reflected image of the blue one will be projected into the eye in the same 

 direction as that of the red image, when we have the sensation of purple.] 



3. A rotatory disc, with sectors of various colours, is rapidly rotated in front of the eyes. On 

 rapidly rotating the coloured disc, the impressions produced by the individual colours are united 

 to produce a mixed colour. If the rotating disc, which yields, let us suppose, white, on mixing 

 the colours of the spectrum, be reflected in a rapidly rotating mirror, then the individual 

 components of the white reapj)ear. 



4. Place in front of each of the small holes in the cardboard used for Scheiner's experiment 



