56o 



NATURE 



[April 13, 1899 



In my earlier experiments the luminous object con- 

 sisted of a small bright disc, but it was soon found 

 desirable to employ a source of light having a more 

 distinctive and conspicuous form than that of a simple 

 circle, and after several trials an excellent object was 

 found in the horseshoe-shaped filament of an electric 

 lamp. The lamp should be dimmed, either by placing 

 coloured glasses before it, or, preferably, by inserting a 

 resistance. An Edison-Swan 5o-volt lamp of 8 candle- 

 power has a suitable filament, and may advantageously 



be connected in series with one or two of the lamps 

 ordinarily used on the circuit. Most electric lamps have 

 looped filaments, and are not so well adapted for the 

 observation. 



When the dimmed lamp is looked at from a distance 

 of a few feet through a lens of about 6 inches focal 

 length, held close to the eye, the patch of luminosity 

 formed upon the retina appears to be made up of a 

 crowd of separate images of the filament, some being 



brighter than others, as represented in Kig. i,' When 

 the observer is near the lamp, the number of images is 

 comparatively small ; as he retires, it gradually increases, 

 but after a certain distance has been reached the 

 definition of the images becomes impaired, and they 

 can no longer be easily distinguished. The method of 

 observation as thus described may obviously be varied, 



' The photogrnphs, Figs, i to 4. 

 (luction. In the ortgin.tls each pictu 

 of separate images of the tilament. 



NO. 1537, VOL. 59] 



too delicate for satisfactory rcpro- 

 s easily seen to be entirely composcii 



and, indeed, an experienced observer can put his ey 

 sufficiently out of focus without the aid of any lens. 



To assist in analysing the luminous field, an adjustabl— 

 slit, taken from a spectroscope, was interposed between 

 the eye and the lens. The appearance presented by the 

 filament when the slit was made i(j inch (o'3 mm.) wide, 

 is very well imitated in Figs. 2, 3 and 4, which show thf^ 

 effect with the slit in hoiizontal, 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 75 meshes to the linear inch, 

 the other 50 ; a slit .^^ inch (i mm.) in width was placed 

 before the lens. 



An attempt was made to count the greatest number of 

 images that could be seen with fair distinctness. The 

 whole of the filament was screened from view except a 

 short portion of one limb, which was viewed from a 

 distance of about 8 feet through a horizontal slit and a 

 lens of 5 inches focus. According to the estimates ot 



several different observers, the number of images was 

 greater than :o and less than 30 (whence it is calculated 

 that without the slit there would be some 50o\ Exact 

 enumeration is perhaps impossible, for though at the 

 first glance one receives the impression that the number 

 is quite definite and probably about 25, closer examin- 

 ation shows that it is often very difl^icult to localise the 

 line of demarcation between successive images. 



If the distance between the eye and the incandescent 

 filament is much more than 8 feet, or if a lens of shorter 



