140 Mr. A. Schuster. Experiments with [June 5, 



For this experiment the end wall was removed from the tail-piece 

 (fig. 3), and a large flexible hoop substituted. By this means, it 

 was hoped that when the whole was placed in the bath it would be 

 possible, by mere expansion of the hoop, to obtain a clean surface in 

 the well. The event proved, however, that the purification did not 

 proceed readily beyond the earlier stages, unless the passage of the 

 contamination through the long channel of the tail-piece was facili- 

 tated by wind.] 



V. " Experiments with Lord Rayleigh's Colour Box." By 

 ARTHUR SCHUSTER, F.R.S. Received May 15, 1890. 



Lord Rayleigh described before the meeting of the British Associa- 

 tion, in 1881,* a colour box in which artificial yellow is produced by 

 mixing a pure red and green, and this yellow is directly compared 

 to the yellow of the spectrum. Lord Rayleigh has given an account 

 of certain peculiarities of vision observed in a number of persons, and 

 it seemed to me worth while to extend the enquiry to a greater 

 number of observers, and also, if possible, to obtain some evidence as 

 to the existence of smaller differences than those described in Lord 

 Bayleigh's paper. 



The instrument used was made according to Lord Rayleigh's 

 second pattern, in which a double-image prism is interposed between 

 the slit and collimator lens ; the prism which separates the light being 

 a direct-vision pi'ism. For the detailed description of the instrument 

 I must refer the reader to Lord Rayleigh's paper. 



My attention was in the first instance directed to prove or dis- 

 prove the existence of small differences in different persons. It was 

 necessary, therefore, only to take persons in whose observing powers 

 I could place some reliance, and, secondly, to multiply the number of 

 observations of each individual, so as to obtain an idea of the degree 

 of accuracy to which the observations could be trusted. The instru- 

 ment was used in a fairly dark room, and the observer was asked to 

 place the Nicol so as to obtain the required match. After the reading 

 had been taken the Nicol was displaced and the observations repeated. 

 Five separate readings were thus generally obtained, and occasionally 

 more. Often separate sets of observations were made for the right 

 and left eye. As, owing to imperfections of construction, the zero of 

 the instrument did not remain constant, either myself or Mr. Hadley, 

 one of the demonstrators in the Phyisical Laboratory, took a reading 

 whenever observations were made. 



I have often compared my vision with Mr. Hadley's, and never 

 detected any difference amounting to more than O'l of a division of the 



* ' Nature,' Nov. 17, 18 1. 



