44 



A POPULAR ACCOUNT 



be such as to put it in a state in 

 which it would be easily transmitted by 

 another similar refracting surface, such 

 as S' S', if that surface received the ray 

 immediately after its passage through 

 S S. But this state of easy transmission 

 does not continue. When the light has 

 arrived at 1 it is in a state of easy re- 

 flection, so that if it were intercepted at 

 1 by such a surface as S' S', it would be 

 reflected back in the direction 1 B A. 

 After passing the division 1, the state of 

 the ray is again changed, and when it 

 has arrived at the division 2, it is again 

 in a state of easy transmission, as at B. 

 If the surface, S' S', therefore, met the 

 ray at 2, the ray would pass freely 

 through it in the direction 3, 4, &c. In 

 passing from 2 to 3 the ray again 

 changes its state, and is found at 3 to 

 be in the same disposition to be reflected 

 as it was at 1 ; and such reflection 

 would, in fact, take place if the surface, 

 S' S', intercepted the ray at 3. In this 

 manner the ray passes alternately into 

 states of easy transmission and reflec- 

 tion, at the successive points, 4, 5, 6, &c. 



(57.) These alternate states of the ray 

 Newton calls fits, the light being in jits 

 of easy reflection at the points 1, 3, 5, 

 &c. ; and in fits of easy transmission 

 at the points 2, 4, 6, &c. The spaces 

 B2, 24, &c., or 13, 35, Sec., he calls 

 the interval of the fits. Although these 

 phrases imply a theory or hypothesis, 

 yet Newton intends them merely as 

 names for effects which are known to 

 exist, and distinctly disclaims the adop- 

 tion of any hypothesis, endeavouring in 

 every case to render his inferences inde- 

 pendent of everything except the result 

 of experiment or observation. 



In describing the phenomena of phy- 

 sical science it is extremely difficult, if not 

 impossible, to avoid expressions and 

 terms which imply a theory or a sup- 

 posed cause for effects. Every writer, 

 but more especially he who promulgates 

 new facts, should be cautious to remind 

 the student that the language of cau- 

 sation, the use of which in physics is 

 inevitable, is nothing more than a me- 

 thod of expressing the classification of 

 effects ; and that, when we are said to 

 "discover the cause" of any appear- 

 ance, nothing more is to be understood 

 than that we have found a class of phe- 

 nomena to which it belongs and must 

 be referred. There is no philosopher 

 who seems more conscious of the neces- 

 sity of this than Newton ; and, accord- 

 ingly, in the introduction of the phraseo- 



logy to' which we have just alluded, he 

 warns his reader that he does not pre- 

 tend to affirm " what kind of action this 

 (the fits) is ; whether it consists in a cir- 

 culating or a vibrating motion of the 

 ray, or of the medium, or of something 

 else. Those that are averse to assenting 

 to any new discoveries but such as they 

 can explain by an hypothesis, may for 

 the present suppose that, as stones, by 

 falling upon water, put the water into 

 an undulating motion, and all bodies, by 

 percussion, excite vibrations in the air ; 

 so the rays of light, by impinging on any 

 refracting or reflecting surface, excite 

 vibrations in the refracting or reflecting 

 medium or substance, and by exciting 

 them agitate the solid parts of the re- 

 fracting or reflecting body, and by agi- 

 tating them cause them to grow warm 

 or hot ; that the vibrations thus excited 

 are propagated in the refracting or re- 

 flecting medium or substance, much 

 after the manner that vibrations are pro- 

 pagated iri the air for causing sound, 

 and move faster than the rays, so as to 

 overtake them ; and that when any ray 

 is in that part of the vibration which 

 conspires with its motion, it easily breaks 

 through a refracting surface ; but when 

 it is in the contrary part of the vibration, 

 which impedes its motion, it is easily 

 reflected ; and by consequence, that 

 every ray is successively disposed to be 

 easily reflected or easily transmitted, by 

 every vibration which overtakes it. 

 But, whether this hypothesis be true 

 or false, I do not here consider. I con- 

 tent myself with the bare discovery that 

 rays of light are, by some cause or 

 other, alternately disposed to be reflected 

 or refracted for many vicissitudes." 



(58.) By the observation of air between 

 glass lenses, Newton ascertained the 

 fact that, after passing through a certain 

 thickness of air, a ray would be reflected 

 or transmitted, according to the degree 

 of thickness of the air and the species of 

 the light. By actual admeasurement he 

 ascertained the least thickness at which 

 each species of homogeneous light 

 would be reflected, a magnitude which 

 will easily be perceived to be equal to 

 half the interval of the fits. We have 

 already observed that this interval is 

 different in different kinds of light, being 

 greater for the less refrangible rays than 

 for the more refrangible. The following 

 Table exhibits the interval for lights of 

 the different degrees of refrangibility. 

 If an inch be supposed to be divided 

 into ten millions of equal parts, the 



