46 Mr. G. W. Wigner on Purification of Sewage. 



Sir David Brewster (Optics, p. 294) has stated that the centre 

 of visual direction coincides with the geometrical centre of the 

 eyeball ; but it has been objected to this, that Volkman's experi- 

 ment on visual direction proves that the lines of visual direction 

 cross each other in a point anterior to the geometrical centre, and 

 that calculation will show this is inconsistent with the curvatures 

 and refractive powers of the humours composing the refractive 

 media of the eye; but as, in the powdered-mirror experiment, 

 the grain of powder seen singly and forming the centre of the 

 system of lines marks the point through which a perpendicular 

 to the surface of the mirror represents the line of visual direc- 

 tion, and as this line is immoveable when the eyeball is moved, 

 it follows that this line passes through a point in the eyeball sta- 

 tionary with regard to the eyeball, notwithstanding the motion 

 of the eyeball ; i. e. wherever the centre of visual direction may 

 be, it is the centre of motion, if not the centre of figure, of the 

 eyeball. 



Madras, May 20, 1868. 



X. On Purification of Sewage. 

 By G. W. Wigner, F.R.A.S* 



ANEW mode of purification of sewage by the addition of 

 chemical agents has been introduced by Mr. Sillar and 

 myself, and some experiments have been tried on a large scale 

 at Tottenham. The process is a cheap one, and, as far as can be 

 judged from the Tottenham experiments, is very successful. The 

 time occupied in precipitation is very short, and the proportion 

 of impurities removed is large. 



A tank holding 5000 gallons was filled with sewage and clarified 

 eight times in succession, the average time occupied in filling 

 and settling being less than twenty minutes. The supernatant 

 water was clear, and almost free from either taste or smell. The 

 analyses below show that more than 80 per cent, of the ammonia 

 originally present in the sewage was carried into the sediment. 

 The produce of the 40,000 gallons was about 8 cwt. of manure, 

 closely resembling guano in appearance. A large tank, holding 

 40,000 gallons, was afterwards filled and precipitated with equally 

 satisfactory results. 



Some more experiments are to take place shortly ; and if these 

 are successful, the details of the process will be published. 



* Communicated by the Author. 



