40 



A POPULAR ACCOUNT 



tenacious or glutinous quality, in virtue 

 of which it may be blown into bubbles, 

 or it may be thrown into that state by 

 mere agitation. Every one is familiar 

 with the various colours which these 

 bubbles reflect. Similar appearances 

 are exhibited by glass when blown into 

 bubbles of sufficient tenuity. Since these 

 effects are not produced when the bound- 

 ing surfaces of the medium are more 

 distant from one another, we are com- 

 pelled to suppose that when the light 

 first enters the transparent medium, it is 

 put into some state in which it does not 

 continue during its entire course through 

 the medium. This inference is as sin- 

 gular and important as it is inevitable. 

 Suppose that the two surfaces of water 

 impregnated with soap were at a dis- 

 tance of one inch asunder a ray of 

 light entering the first surface perpen- 

 dicularly, would penetrate the water, and 

 passing through the second surface, 

 would issue from the water at the other 

 side, preserving its original direction. 

 Now, suppose that the second surface 

 of the water, instead of intercepting the 

 course of the ray at the distance of an 

 inch from the first surface, meets it at a 

 distance from that surface, equal to the 

 thickness of a certain part of the soap- 

 bubble, to which we have alluded the 

 ray will no longer be allowed to pass 

 out in its original state at the second 

 surface. On the contrary, if it be white 

 solar light, that part of it which has a 

 certain colour, say red, will be reflected 

 back in the direction from which it 

 came, while the remainder only of the ray 

 which, combined with red, would pro- 

 duce white, will be transmitted. It there- 

 fore follows that, in this instance, after 

 the ray has penetrated the water through 

 a space equal to the supposed thickness 

 of the bubble, that portion of it which is 

 red is put into such a state, that were it 

 to encounter the second surface, it could 

 not penetrate it, and would be reflected. 

 This state, however, does not continue ; 

 for when the white ray is allowed to 

 proceed further into the water before it 

 is intercepted by the second surface, it 

 will be brought into a state in which it 

 will penetrate that surface, and be trans- 

 mitted into the ambient medium. Such 

 is an example of the class of facts which 

 form the basis of the experiments and 

 investigations which we are now about 

 to explain. 



(52.) The property which we have in- 

 stanced in water and glass is common to 

 all transparent media. The evanescent 



and fluctuating nature of a water-bubble 

 renders it an inconvenient object of ex- 

 perimental inquiry. Glass is better, but 

 still is difficult to procure, and to retain 

 in the highly attenuated state which is 

 necessary to manifest the desired effects. 

 By the following contrivance, Newton 

 rendered air, though at the first view 

 an unpromising agent, available for the 

 purposes of deliberate and close experi- 

 mental observation. 



He procured a double convex lens, 

 the object-glass of a fifty foot telescope, 

 and consequently having a degree of 

 convexity so small as to be scarcely per- 

 ceptible. On this he placed the plane 

 surface of another lens, so that the two 

 surfaces were in absolute contact at the 

 centre, the distance between them in- 

 creasing with the increased distance from 

 that centre. A lens of air was thus in- 

 closed between two glass lenses. This 

 air lens was plane on one side and con- 

 cave on the other, losing all thickness 

 at the centre at which may be conceived 

 an infinitely small space, filled by the 

 point of contact of the glass lenses. 

 Taking this point of contact as a centre, 

 let us suppose a number of concentric 

 circles to be traced on the lenses as 

 represented in fig. 

 41. Let the'smallest 

 circle be called 1 , the 

 next 2, the next 3, 

 and so on. It is 

 plain that the thick- 

 ness of the air under 

 the circle 1, is less 

 than under the circle 



2. In like manner 

 the thickness under 



the circle 2 is less than under the circle 



3, and so on. A section of the lenses 

 exhibiting the thickness of the air be- 

 tween the lenses under the several circles 

 1, 2, 3, &c.is represented \r\fig. 42. 



Fig. 42. 



Upon exposing these lenses to a beam 

 of light, a very minute black spot was 

 observed at the centre. Immediately 

 around this circular spot was a ring of blue 

 colour, which gradually emerged from the 

 black, so as to assume the perfect blue 

 tint at some distance from the black 

 spot. This blue ring was surrounded 

 with a white one, into which it in like 



