THE REFRACTING TELESCOPE. 173 



stick. Although many of the properties of the light-waves are also 

 common to all forms of wave-motion, yet others are distinctively due 

 to the waves being of this particular kind. This form of wave, there- 

 fore, is to be carefully distinguished from that propagated in a fluid, 

 where there is always a forward and backward motion to the par- 

 ticles. For example, in the familiar case of waves on the surface of 

 water, the particles of water move in circular paths as the waves pass 

 \yj — that is, each particle moves forward and back exactly as far as 

 it moves up and down. Also in the case of sound-waves, which are 

 waves propagated through a gas, the particles of the air move only 

 forward and back along the line in which the sound-waves are ad- 

 vancing. 



The diffraction grating shows that the waves which produce the 

 sensation of light are very minute, and are of every possible length, 

 between the limits of 32,000 to the inch to 64,000 to the inch, meas- 

 ured from crest to crest. This is only one fifth of the total range of 

 wave-lengths that have been measured radiating from the sun, but 

 only those longer than yj^^ir of an inch, or shorter than -g-r^oTr of an 

 inch, ordinarily reach the retina to produce the sensation of light. 

 The diffraction grating also shows that the color of light is due di- 

 rectly to the length of the waves, the longest producing the sensation 

 of red light, the shortest of violet, while ranged in between come the 

 various shades of orange, yellow, green, and blue. 



Diagram 1 will perhaps give a better idea of the true size and 

 number of the light- waves than is possible from a mere statement of 

 their length and velocity in figures. It represents in section, magnified 

 five hundred diameters, a series of crests of the longest waves that 

 affect the eye as light, passing through a hole in writing-paper, pricked 

 by an ordinary No. 12 sewing-needle, measuring one seventy-fifth of 

 an inch in diameter. It will be noticed that, although the magnified 

 diameter of the hole appears nearly seven inches across, yet the equally 

 magnified crests of the light-waves are still only just far enough apart 

 to be distinctly separated by the eye. On this scale the pupil of the eye 

 would appear nine feet across, and a very good idea of the number of 

 these particular waves, which enter the eye in a continuous stream 

 whenever it receives the light of a distant object, can be had by con- 

 sidering that, if every one of these light-waves passing through the 

 needle-hole in a single second had been represented on the diagram, 

 one behind the other, they would have formed a band extending in 

 the direction of the arrow to a distance of nearly 100,000,000 miles, 

 and to have shown them all on the diagram would have necessitated 

 the paper being long enough to reach from the earth to beyond the 

 sun ! 



Having once established the fact that the sensation of light is 

 caused by waves originated in the sun and stars, falling upon and irri- 

 tating the retina of the eye, it of course follows that space must be 



