339 



at regular intervals from each other, then the rays, which emerge, 

 and after reflection re-enter the same surface, will also be separated 

 so as to present streaky appearances, in some of which the order of 

 the colours will differ from that in others, agreeably to observation. 



With respect to the curvature of the prismatic bows, that mani- 

 festly depends upon the position of the eye, as the lines of equal in- 

 cidence form a cone, of which the eye is apex ; and, accordingly, as 

 the eye recedes from the prism, the bow becomes less curved, and 

 for a small space will appear straight. 



Dr. Herschel having remarked, when a plane surface is applied to 

 a prism, that the appearance of the streaks that are seen in contact 

 with the prismatic bow depends on what he has termed the critical 

 separation of the rays, infers, that those rings of colours which are 

 seen when a lens is laid upon a plane surface, depend also upon the 

 same critical separation ; the lens being in this case considered as a 

 prism bent round into a circular form. 



The several points of resemblance in the circumstances under 

 which the bow-streaks from a prism, and the rings from contact of 

 lenses, appear or disappear, or change their form or their colours, 

 are next compared ; and are considered by the author as decisive 

 proof that each arise from the same critical separation at the boundary 

 of prismatic reflection : and if the mere difference of refrangibility of 

 the several colours is sufficient to account for the phenomena, it is 

 inferred, that no alternate fits of easy reflection and easy transmis- 

 sion really exist as original properties in the rays of light. 



An Account of a Calculus from the Human Bladder of uncommon Mag- 

 nitude. By Sir James Earle, F.R.S. Read June 15, 1809. [Phil. 

 Trans. 1809,^. 303.] 



This calculus, which is considered as the largest on record, weighs 

 44 ounces avoirdupois. It was taken after death from the bladder 

 of Sir James Ogilvie, who had submitted to a fruitless attempt to 

 remove it by the usual operation of lithotomy, rather than prolong 

 an existence extremely miserable from this among other conse- 

 quences of a blow on his back thirty years before. The stone so 

 completely filled the cavity of the bladder, that it was with difficulty 

 taken out, although there was no real adhesion. Its texture was 

 less compact than that of calculi in general, but agreed in appear- 

 ance with that species which has been called the Fusible Calculus ; 

 and it was ascertained by Dr. Powel to contain the same ingredients 

 as it was composed of, the ammoniacal phosphate of magnesia, with 

 phosphate of lime. 



On expectorated Matter. By George Pearson, M.D. F.R.S. Read 

 June 15, 1809. [Phil. Trans. 1809, p. 313.] 



Dr. Pearson's inquiry comprehends several varieties of expectorated 

 matter, which he arranges under the following heads : 

 z 2 



