367 



tween the two adjacent surfaces, it may increase the extent in any 

 greater proportion. 



The last objection to which the author replies, relates to those po- 

 sitions in which rings of colours, and other similar phenomena, are 

 seen, but in which the colours produced by critical separation could 

 not reach the eye. For instance, rings and bands of colour, which 

 arise from contact at the under surface of a plate of glass terminated 

 by parallel planes, are seen through the upper surface, although co- 

 lours separated by critical reflection or intermission, evidently could 

 not come to the eye under these circumstances. But Dr. Herschel 

 reminds us, that he does not affirm critical separation to be the sole 

 cause of the rings produced by contact of a plane and sphere, but 

 that it only furnishes the colours, which are afterwards modified by 

 the subjacent spherical surface ; and next proceeds to several sets of 

 experiments, which he considers decisive in support of the validity 

 of his theory, in reply to this objection. 



In the first set of experiments a series of prisms, of different forms, 

 are successively placed within their bases, resting upon a spherical 

 metallic surface. 



When a right-angled prism was placed in this situation, and the 

 eye was gradually raised from a level with its base, no colours were 

 seen till it arrived at the elevation necessary for critical separation. 

 At this point the blue bow became visible, and rings began to be -per- 

 ceived at the same time. When the eye was lifted gradually higher 

 and higher, till it arrived opposite to the vertex of the prism, the 

 rings continued visible, without interruption, notwithstanding suc- 

 cessive changes which occurred in their colours and size : and even 

 when the eye was carried beyond the vertical position, the same rings 

 continued visible, so as to be seen, upon the whole, through a range 

 of at least 77. 



Instead of the right-angled prism, having a refracting angle of 45 

 on each side, Dr. Herschel afterwards substituted prisms with their 

 vertical angles successively more obtuse, and with equal refracting 

 angles on each side, first of 30, then of 25, then of 20, and lastly 

 of 9 on each side. 



In all these instances the phenomena were similar ; but the range 

 of visibility increased in proportion as the refracting angle was 

 smaller, so that in the last instance the range within which the rings 

 were visible from each surface, exceeded 138. And hence might be 

 inferred the still greater extent, in case of plain glass, which may be 

 looked upon as a prism with a vanishing refracting angle. 



These experiments, in Dr. Herschel's estimation, establish the mo- 

 difying power of spherical surfaces, whereby they render colours that 

 have been entirely separated, visible in every direction. 



In the next set of experiments, the author substitutes a cylindrical 

 surface for that which in the former set was spherical ; and by a si- 

 milar series of prisms, successively more and more obtuse at their 

 vertical angles, the coloured streaks, which in this case appeared 

 instead of rings, were rendered visible to greater and greater extent, 

 till with a plain glass they were seen as far as 1 70. 



