1894] 



Images following Visual Impressions. 



135 



employed. With white electric light the colour of the ghost was 

 violet. 



This method of experimenting was, however, found to be unsuited for 

 the purpose in view, and I mention it only on account of the facility 

 which it affords for exhibiting the phenomenon to a large number of 

 persons. To obtain results of any value, it was necessary to employ 

 the simple colours of the spectrum, and the arrangement finally 

 adopted for this purpose is indicated in fig. 1. L is a lantern con- 



Fia. 1. 



-/38m. 



taining a high-pressure oxy hydro gen light, which is better adapted 

 for the experiment than an arc lamp, the intensity of the light being 

 easily varied. S is an adjustable slit, M a projection lens, P a bi- 

 sulphide of carbon prism, D a metal plate, in the middle of which is 

 a circular aperture 2 mm. in diameter. A spectrum, 6 or 7 cm. in 

 length, can be projected upon D, a small selected portion of it passing 

 through the aperture and falling upon the mirror Q, which is 8 cm. 

 in diameter. To the back of the mirror is attached a horizontal arm, 

 which is not quite perpendicular to the mirror, its inclination being 

 capable of adjustment. The arm is rotated by clock-work, and turns 

 once in 1| sees. 



It was at first attempted to study the phenomenon by direct eye 

 observations of the reflected image of the aperture in the rotating 

 mirror, the aperture being covered by a piece of finely-ground glass ; 

 but, for pretty obvious reasons, no satisfactory results could be thus 

 obtained. 



A telescope was then employed, having a power of 12, and an eye- 



