244 Mr. Shelford Bidwell. 



being brighter than others, as represented in fig. 1. The number was 

 apparently few when the observer was near the lamp, and greatly 

 increased as he retired from it, or moved the lens further from his eye. 



It occurred to me that the analysis of the luminous field would be 

 facilitated if the attention could be confined to a small portion of it. 

 With this object in view I interposed an adjustable slit taken from a 

 spectroscope, between the eye and the lens, and adjusted its width by 

 trial. When the round hole in the brass plate was viewed through this 

 arrangement it appeared like a string of bright beads, arranged not in 

 a perfectly straight line but somewhat sinuously. A slight movement 

 of the slit in a direction perpendicular to its length produced a curious 

 wavelike motion of the beads. 



By sufficiently increasing the distance between the source of light 

 and the eye, perhaps as many as twenty-four or twenty-five bright 

 spots might be made to appear in the row, but they could not be 

 counted with certainty. At a greater distance, or with a lens of shorter 

 focus, the spots became indistinct and blurred. 



The appearance presented by the filament of the electric lamp, when 

 seen through the slit, made -$ inch (O3mm.) wide, and a convex lens of 

 5 inches (12-7 cm.) focus is very well imitated in figs. 2, 3, and 4, which 

 show the effect when the slit is in horizontal, vertical, and intermediate 

 positions. The imitation was produced by photographing the lamp by 

 means of a lens covered with two layers of gauze, the one containing 

 seventy-five meshes to the linear inch, the other fifty ; a slit ^ inch 

 (1 mm.) in width was placed before the lens. 



Another attempt was made to count the number of images in a row. 

 The whole of the filament was screened from view, except a very short 

 portion of one limb, which was viewed from a distance of about 8 feet 

 (2 '5 m.) through the spectroscope slit, and a convex lens of 5 inches 

 (12*7 cm.) focus. A sheet of coloured glass was interposed as before, 

 and care was taken to hold the slit in such a position that the length 

 of the row was a maximum. According to the estimates of five 

 different observers, the number of images ranged from twenty to 

 thirty. One excellent observer counted them several times, his 

 greatest total being twenty-seven, and his smallest twenty-three. 

 Exact enumeration is perhaps impossible, for though at the first glance 

 one receives the impression that the number of images is quite definite, 

 and probably about twenty-five, closer examination shows that it is 

 often very difficult to localise the line of demarcation between successive 

 images. 



The number of images in a row varies with the dilatation of the 

 pupil. If a lighted candle be held near the eye with which the obser- 

 vation is not being made, the pupil of the observing eye contracts 

 sympathetically, and two or three images disappear from each end of 

 the row. 



