796 



While considering the theory of Professor Powell's bands, the au- 

 thor was led to perceive the explanation of certain bands, described 

 by Professor Powell, which are seen in the secondary spectrum formed 

 by two prisms which produce a partial achromatism. Although the 

 account of these bands has been published many years, they do not 

 seem hitherto to have attracted attention. It is easily shown by com- 

 mon optics that when two colours are united by means of two prisms, 

 the deviation, regarded as a function of the refractive index, the 

 angle of incidence being given, is a maximum or minimum for some 

 intermediate colour. For the latter colour, two portions of light of 

 consecutive degrees of refrangibility come out parallel ; and there- 

 fore the diffraction bands belonging to different kinds of light, of 

 very nearly the same refrangibility with the one in question, are su- 

 perposed in such a manner that the dark and bright bauds respect- 

 ively coincide. Thus distinct bands are visible in the secondary 

 spectrum, although none would be seen in the spectrum formed by 

 a single prism, in consequence of the mixture of the bright and dark 

 bands belonging to different kinds of light of nearly the same degree 

 of refrangibility. The diffraction bands here spoken of are of very 

 sensible breadth, in consequence of the small width of the aperture 

 employed in the actual experiment. 



When a spectrum is viewed through a narrow slit half covered by 

 a plate of mica, the edge of which bisects the slit longitudinally, and 

 is held parallel to the fixed lines of the spectrum, the bands described 

 by Sir David Brewster are seen, provided the mica plate lie at the 

 side at which the blue end of the spectrum is seen, and provided the 

 thickness of the plate and the breadth of the slit lie within certain 

 limits. When these bands are invisible in consequence of the slit 

 being too narrow, or the spectrum too broad, it follows from theory 

 that the bands ought to appear when the slit and plate are turned 

 round the axis of the eye, so that the edge of the plate is no longer 

 parallel to the fixed lines of the spectrum. The author has verified 

 this conclusion by experiment, employing plates adapted to obser- 

 vations with the naked eye, which are best suited to the purpose. 



February 1, 1849. 

 GEORGE RENNIE, Esq., Treasurer, Vice-President, in the Chair. 



The following paper was read :— "On the Chemistry of the Urine ;" 

 in three Parts. By H. Bence Jones, M.D., M.A., F.R.S. 



Part I. On the variations of the Acidity of the Urine in Health. 



The mode of examination adopted by the author was the following : 

 Two test solutions were made ; the one with carbonate of soda ; the 

 other with dilute sulphuric acid, of such strength that each measure 

 of a graduated tube, when filled with either solution, was equivalent 

 to one-twelfth of a grain of dry and pure carbonate of soda. 



A weighed quantity of urine was neutralized by one or other of 



