214 



SCIENCE. 



[VoT,. II., No. 29. 



THE CONDITIONS NECESSARY FOR 

 THE SENSATION OF LIGHT. 



It is general!}' assumed that the onlj' condi- 

 tion necessarj' for the production of the seu- 

 sation of light by the action of radiant energy 

 is, that the radiant energy must be of a certain 

 wave-length within the limits of wave-length 

 of the visible spectrum, namel}-, between wave- 

 lengths 7.604 xlO-^"* centimetres and 3.933 X 

 10"^ centimetres ; that, when the eye perceives 

 notliino', none of these wave-lengths can be 

 present. It is worth while, therefore, to ex- 

 amine those phj'sical conditions that result in 

 giving the sensation of light to ascertain 

 whether such assumption is warranted. As to 

 the eye itself, it will not make any diiference 

 so far as this question is concerned, whether one 

 accepts the Young- Helmholtz theory of vision, 

 the Herring theory, or any other. The only 

 important fact is, that, in either, energy is re- 

 quii-ed and is expended in the eye ; but it is 

 important to know how to measure the energy, 

 and to have a tolerably clear idea about its 

 form. Without any question, a ray of radiant 

 energy, such as is emitted by a heated molecule 

 or atom of hydrogen, consists of a single line 

 of undulations of a definite wave-length, for the 

 molecule cools (that is, loses its heat-energy) 

 by imparting it to the ether ; and a ' wave- 

 length ' is simply the distance to which such 

 a dTsturbance in the ether will be propagated 

 during the time of a single vibration of the 

 molecule. As each vibration of the latter 

 imparts some of its energy to the moving 

 ether, it follows that the energy of a ray of 

 light must depend upon the number of vibra- 

 tions per second ; or, what is the same thing, 

 the energy of the ray is proportional' to its 

 length. As all rays move with the same 

 velocity in the ether, it follows that any object 

 that should receive such radiant energy would 

 receive an amount proportional to the time. 



Suppose, now, that an atom of hydrogen be 

 made to vibrate, no matter how, so as to give 

 a wave-length C=6.562 X 10"^ centimetres. If 

 such a ray falls upon the eye, it will produce 

 the sensation of redness, and, if the eye re- 

 ceives the vibrations for one second, it will 

 receive 4.577 X 10" vibrations ; that is to say, 

 it will receive as many undulations from the 

 ether as the generating atom made in the 

 interval of one second. Now, we know experi- 

 mentally that the eye can perceive when the 

 interval is as small as the millionth of a sec- 

 ond, when the number of vibrations of such a 

 ray as the above would be 4.577x10*, a very 

 respectable number. It would seem probable 



that that number might be cousiderahly re- 

 duced, and still leave a suflioient numlier to 

 aflect the eye. If the time-interval should be 

 made so short as the one ten-billionth of a 

 second, there would then be 45,770 such undu- 

 lations that would enter the eye. But there 

 must be a limit to the number needed to pro- 

 duce the sensation; and it is also probable 

 that this limit will differ in different persons. 

 Admitting this time-limit, it follows that undu- 

 lations of proper wave-length may exist about 

 us, and yet not be sufficient in time-quantity 

 to affect the eye. If other vertebrates or in- 

 sects possess a shorter limit than man, it is 

 certain that they will see when man cannot. 

 But the energy of vibrations varies as the 

 square of the amplitude ; and hence, if one of 

 two rays of equal length has a greater ampli- 

 tude than the other, the latter might be seen, 

 while the former might not, although they had 

 the same wave-length. 



According to the kinetic theorj- of gases, 

 the molecules are in incessant motion, in 

 which coUisions result in changing the direc- 

 tions of the free paths of each of the mole- 

 cules, and also in making each to vibrate, 

 because molecules are elastic. This vibratory 

 motion proper, being a change of form of the 

 molecule, is what constitutes its heat-energy. 

 The interval between encounters gives oppor- 

 tunity to each molecule to vibrate in its own 

 periodic time or some of its harmonics. Max- 

 well computed the number of impacts per 

 second for several gases,' and gives, for hydro- 

 gen, 17,750 X 10". If, then, we divide the num- 

 ber of vibrations per second by the number of 

 impacts, we shall have the number of vibra- 



.• w ■ f 4. 577X10^ „, ..„ 



tions between impacts: -;= 2o,i{)i). 



177.50x10° 



This is on the supposition that the vibrations 

 produced are all of the wave-length of the C 

 hydrogen-line. 



It is iiighly probable that this hydrogen-line 

 is not due to the fundamental vibrations of 

 the hydrogen molecule, but that it is some 

 harmonic (the twentieth, according to Stoney). 

 Whatever its harmonic relation may be, it 

 must be highly probable that it will fVequentl\- 

 be produced when the conditions are as they 

 are in ordinary gas ; but, in normal conditions 

 as to temperature, that gas is not luminous. 

 If tins reasoning be right, the reason it is not 

 luminous at ordinary temperatures and press- 

 ures is due solely to the slight amplitude of 

 the vibrations of proper wave-length, not to 

 their entire absence. When the gas is heat- 



1 Katun, Sept. 25, 1873. 



