NATURAL PHILOSOPHY. 159 



ed little attention. It "was believed by physicists that they were due 

 to the minute imperfections of the glass of the prism, the little irregu- 

 larities along the edge of the slit through which the light is admitted 

 into the dark room, or to other similar causes ; and that they were in 

 no way related to the constitution of the light itself. 



The numerous experiments which I have made in connection with 

 this subject, have led me to the conviction that the longitudinal lines 

 are not due to the irregularities alluded to, but are produced by inter- 

 ference. Whoever is accustomed to experiments on light, will find 

 the mere inspection of these lines sufficient to convince him that they 

 are due to no mechanical cause. The clearness and beauty with 

 which they exhibit themselves, and their sharp and definite character 

 throughout their entire length, distinguish them at a glance from those 

 which might be produced by unevenness of the slit's edge, particles 

 of dust, imperfections of the apparatus, and so forth. 



In the first place, I have observed that the longitudinal lines are 

 entirely absent when a large lens is not applied, and when it is placed 

 close to the prism and at right angles to the rays issuing from the 

 same. I have further seen, that the lens changes the breadth of the 

 spectrum only, and not its length. 



Thus in one of my experiments, which was conducted with an equi- 

 lateral vertical prism and a biconvex lens of 90 centimetres focal dis- 

 tance, after ascertaining by trial the position in which the spectrum 

 was most clearly shown, I found its dimensions to be 



Length 13-4 centirns. 



Breadth 3-2 " 



The lens was then removed, and the position of the screen and 

 prism remaining unchanged, the dimensions were found to be 



Length 13-4 centims. 



Breadth 15-8 " 



Hence the introduction of the lens caused the disappearance of 12-6 

 out of 15-8 parts of the .spectrum; the light must have been com- 

 pressed from a space of 15-8 to a space of 3-2. 



In the latter space, the rays which had passed through the lens 

 overlaid each other, as may be rendered evident by a very simple 

 experiment. It is only necessary to move a bit of cardboard close to 

 the lens from top to bottom, or the reverse, and thus to receive a por- 

 tion of the rays passing through it. It is then seen, that no matter 

 how great the portion may be which is thus intercepted, the dimen- 

 sions of the spectrum remain unaltered, its brightness alone being 

 more and more diminished as the intercepted portion becomes greater. 

 This experiment establishes the fact of superposition, and the produc- 

 tion of the longitudinal lines by interference is a simple result of this. 



It is really interesting to observe how every line may be caused to 

 vanish by moving the card in a proper manner before the lens. From 



15 



