44 Mr. J. C. Douglas on an Optical Experiment. 



If (the appearance described above being visible) the observer 

 move his head with relation to the glass, the centre from which 

 the lines appear to radiate and the whole system of lines move. 

 If, however, the observer move his eyeball in its orbit, the glass 

 and his head being stationary, the centre and system of radiating 

 lines do not appear affected. On regarding the prepared glass 

 with both eyes, he will see two centres, from which radiate two 

 systems of lines, one centre being opposite to the image of the pupil 

 of each eye. The appearance is not so distinctly seen, however, 

 with two eyes as with one ; and if one eye be stronger than the 

 other, i. e. more effective for purposes of vision when both are 

 open, the system of lines appertaining to the weaker eye is less 

 distinct than that appertaining to the stronger. The superior 

 distinctness of the appearance when one eye only is used is due 

 to the fact that the eyes mutually interfere when used together; 

 for it is not necessary to shut one eye, but merely to interpose a 

 sheet of card or other opake body between it and the mirror. I 

 have never failed to distinguish the radiating appearance in my 

 own case, nor in the many cases in which I have endeavoured to 

 show it to unscientific persons ; and I believe, if only one eye be 

 used at first, and the instructions given above be strictly adhered 

 to, the appearance may be readily seen by any person ; but as I 

 have heard of an instance in which the phenomenon could not 

 be distinguished, I have thought it necessary to give rather mi- 

 nute instructions*. 



The phenomenon is so easy of explanation that it is not neces- 

 sary I should explain it, I will therefore confine myself to point- 

 ing out that the lines seen represent the apparent directions of 

 the lines joining the particles of powder and their images, and it 

 is evident that the centres of the systems of lines are in the per- 

 pendiculars to the surface of the mirror drawn through the 

 centres of visual direction. The apparatus may be modified in 

 accordance with the wants of each observer, the principle upon 

 which its construction is based remaining the same ; the glass 

 may be of any attainable thickness ; a glass plate may be placed 

 in front of the mirror at various distances from it to carry the 

 powder; engraved Hues, dots, &c. may be employed in lieu of 

 powder ; figures or other characters may be written on the mirror 

 or glass plate if required ; and the whole instrument may be 

 suitably mounted. 



The applications of the instrument described appear numerous ; 

 a few are given below: — Experimental proof is afforded of the prin- 



* The effect was most striking when the powder was very thinly and 

 uniformly spread over a piece of plane glass placed before the mirror, at 

 such a distance that the powder was rather less than half an inch from the 

 reflecting surface. 



