the received Theory of Newton's Rings. 4-61 



through the side of the prism turned towards the room, ap- 

 peared like an uninterrupted and very bright opening passing 

 through both glasses, while the surrounding portion of the 

 base of the prism was dull and dim. This experiment proves 

 the uninterrupted passage of the rays through the centre 

 where the light falls from above upon the uncovered side of 

 the prism. 



When two lenses placed together were laid upon white 

 paper, the hue of the central spot appeared less intense than 

 when black or merely dark-coloured paper was used, and the 

 spot was not at all visible except when the light was obliquely 

 reflected. At smaller angles of incidence it vanished totally, 

 and appeared as a white circle when I looked vertically down 

 upon the place of contact of the glasses, the ring-system being 

 visible at the same time. If, however, the darkness of the 

 central spot was caused, as heretofore supposed, by inter- 

 ference, by a self-destruction of the light, the spot must con- 

 tinue dark, even when the glasses are laid upon a white ground, 

 under all possible angles of incidence, and the light proceed- 

 ing upwards from the paper could not cause the place of con- 

 tact to appear white, as it actually does. 



Having united the summits of two convex lenses with Ca- 

 nadian balsam, I laid them on a black or dark-coloured sur- 

 face, and observed, by all incidences of the rays, that the place 

 on which the balsam lay exhibited the same deep black as 

 when two glasses are pressed together in daylight. When, 

 however, the lenses were laid upon white paper, the black, as 

 in the case of the pressed glasses, was less intense., and the 

 spot also disappeared when small angles of incidence were 

 used. Although the continuity of the glasses in this case was 

 only partially effected by the balsam, still every experiment 

 made with the lenses thus united pronounced decisively that 

 the darkness of the central spot is to be referred exclusively 

 to the free transmission of the light at the place of contact. 



In the first experiment, it is observed that in the neighbour- 

 hood of total reflexion, on the base of the prism, the central 

 spot is not surrounded by rings. Rings cannot show them- 

 selves, because at the place where they ought to appear no 

 rays are transmitted which after reflexion from the lens could 

 interfere with those reflected from the base. The central spot, 

 however, appears black, because as no air exists between the 

 glasses at the point of contact, no total reflexion is here pos- 

 sible, the light being permitted to pass on. For were we to 

 assume the existence of a layer of air at the place of contact, 

 the light must undergo total reflexion at this point also, and 

 hence the central spot must appear as bright as the surround- 



