94 SCIENTIFIC RECREATIONS. 



other philosophers tell us, of the luminous atoms are communicated to this 

 ether, or propagated through it in waves ; these waves enter the pupil of the 

 eye, and strike upon the retina. The motion is thus communicated by the 

 optic nerve to the brain, and then arises the great primary faculty, Con- 

 sciousness. We see light, the waves of which, or ether vibrations, are 

 transversal ; air waves or sound vibrations are longitudinal. 



We have spoken of radiant heat. Light acts in the same way through 

 the ether; and when we consider Sound we shall learn that a certain number 

 of vibrations of a string give a certain sound, and the quicker the vibration 

 the shriller the tone. So in light. The more quickly the waves of luminosity 

 travel to our eye, and the faster they strike it, the greater the difference in 

 the colour, or what we call colour. Light as we see it is composed of 

 different colours, as visible in the rainbow. There are seven primary colours 

 in the sunlight, which is white. These can be divided or " dispersed," and 

 the shortest rays of the spectrum are found to be red, the longest violet. It 

 has been calculated that 39,000 red waves make an inch in length. Light 

 travels at a rate of nearly 1 90,000 miles a second, so if we multiply the 

 number of inches in that distance by the number of red waves, we shall have 

 millions of millions of waves entering the eye in a single second of time. 

 The other waves enter more rapidly still, and " the number of shocks cor- 

 responding to the impression of violet is seven hundred and eighty-nine 

 millions, of millions " per second ! Or taking the velocity of light at 1 86,000 

 miles in a second, it would be six hundred and seventy-eight millions of 

 millions (Tyndall). There may be other colours which we cannot see because 

 the impressions come too rapidly upon the retina; but the violet impression 

 has been thus accurately determined. See page 168. 



We have seen that heat is a kind of motion of particles in a body a 

 vibratory motion which, instead of being apparent to the ear, is apparent 

 to the eye in rays of light. Thus heat, sound, and light are all intimately 

 connected in this way. We have also learnt that rays of light radiate and 

 travel with tremendous speed to our eyes, but without any shock. There is 

 no feeling connected with the entrance of light to the eye any more than 

 there is any sensation of sound when entering the ear, except when the light 

 is vividly and very suddenly revealed, or when a very piercing sound is heard. 

 Then the nerves are excited, and a painful sensation is the result ; but under 

 ordinary circumstances we are not physically conscious of the entrance of 

 light or sound. 



Heat and light are considered to be one and the same thing in different 

 degrees of intensity. The sources of light are various. The sun and fixed 

 stars, heat, electricity, many animals, and some plants, as well as decaying 

 animal matter, give out light. There are luminous and non-luminous bodies. 

 The moon is non-luminous, as she derives her light from the sun, as does the 

 earth, etc. 



Light is distributed in rays. These rays are straight in all directions, 

 The velocity of light is almost inconceivable. It travels at a rate of 186,500 



