36 REPORT — 1873. 



Light. 



Experiments on Liyht with circularhj ruled plates of Glass. 

 Bij Philip Braham, F.C.S. 



A point of lig'tt, viewed at a distance throiio;]! plates of glass •with ooncentric 

 circles ruled tliereou, is seen to be suiTounded b}' rings of brilliant colours. The 

 author tried the experiment of introducing the ruled glass into a beam of sunlight 

 ^ an inch in diameter, and viewing the rings on a screen placed 10 feet from the 

 ruled plate, with the following results : — 



With 2500 lines to the inch there appear two rings of colour, the diameters of 

 the red rings (which are always outwardj being 1 foot 5 inches and 2 feet 10 inches, 

 the width of the rings from the outside of the red to the inside of the violet in each 

 case being respectivelv 31 inches and 6} inches. 



With 3500, 1 foot 8 inches and 3 feet 3 inches, width 4^" and 8". 



With 5000, only one ring 3 feet in diameter, width 8". 



With 10,000, one ring 5 feet in diameter, width 11". 



There are other rings visible, but they are faint and indefinite. 



The coloured rings are also seen by reflection from the outer glass, with the same 

 angulai" dispersion. 



On some Abnormal Effects of Binocular Vision. Bij W. S. Davis. 



While using a Herapath blowpipe a short time since, and having my eyes fixed 

 intently on a bead held in the flame, I was suddenly startled by seeing the papered 

 wall, which was about three feet in front of me, make its appearance close up to 

 the point of the flame, the patterns of the paper being at the same time much 

 diminished in size. Casting my eyes from side to side, and upwards and down- 

 wards, the appearance still remained as distinct as in ordinary sight ; on moving 

 my eyes beyond the boundary of the wall the appearance immediately vanished. 

 I afterwards succeeded in reproducing the appearance by simply looking at the wall 

 and converging the optic axes of my eyes. 



It occurred to me that the phenomenon I had seen was due to the crossing of the 

 optic axes of my eyes, the angle being such that each eye received the impression 

 of a precisely similar figure. Under these circumstances a single figure would be 

 seen, as when a single flat object is viewed with both eyes in ordinary sight. In 

 order to satisfy myself that this was the correct explanation I made a geometrical 

 construction, traced the relations which should hold, and verified them by actual 

 measurements. 



Continuing my experiments, I succeeded by a further convergence of the optic 

 axes to combine alternate patterns, and pairs stiU more widely separated, up to 

 twelve or more. It is a very interesting experiment to combine a given pattern 

 with, say, the fifth or sixth from it, and then by a peculiar effort, more easily made 

 than described, to let one pattern slip at a time, the waD retreating by steps as each 

 pattern is slipped. 



On one occasion, when I had combined two patterns at some distance apai-t, 1 

 happened to shut one eye, when, to my surprise, the combinational figure remained 

 as distinct and at the same distance as before. I can only account for this by sup- 

 posing that the muscles of the eye which was closed were still acting in sympathy 

 with those of the open eye ; and subsequent experiments favoured this view. 



The results of the foregoing experiments led me to think that it would be possible 

 to optically combine two patterns without crossing the optic axes, provided the 

 distance between the centres of the patterns was not greater than that between the 

 centres of the eyes. This I succeeded in doing, and the result was very remarkable : 

 the wall appeared to retreat and take up a fixed position at some distance beyond its 

 actual position; on looking slowly upwards and sideways along the wall" the di- 

 mensions of the room ajjpeared to be enormously increased, while on looking down- 

 wards I appeared to be perched on a sort of gallery, the wall appearing to be several 

 yards from me, and descending many yards below tlie floor on which I was standing. 

 This appearance was as \ivid and distinct as in the case of ordinarv vision. 



