226 



Capt. W. de W. Abney. 



[June 21, 



In order to get a stripe of white inclosed between two saturated 

 stripes of colour a Yernon Harcourt screen was employed instead 

 of a rod. The principle of this may not be known generally, so a 

 brief description of it may be necessary. It consists of a rectan- 

 gular metallic sheet of about two inches wide, in which two broad 

 slits are cut and separated from each other by the width of the slits. 

 This sheet, if placed in the path of the beam allows two stripes of 

 colour and of white to pass. By carefully adjusting the position of 

 this screen a stripe of white may be enclosed between two stripes 

 of colour. The results are given in Table II. 



Table II. Saturated Background. 



The contrasts with gas light, using the same light to dilute a 

 spectrum colour in instrument No, 1, were also measured, and these 

 are given in Table III. 



Table III. Contrasts in Gaslight. 



