Mr Aston, Neon Lamias for Stroboscopic Work 305 



the real efficiency of the discharge in Neon, is not markedly greater 

 than that in e.g. mercury vapour, but the apparent efficiency is 

 enormously enhanced by the fact that it consists so largely of red 

 light. Victor Henri and J. L.desBancels have shown (" Photochemie 

 de la Retine," Jl. Phys. Path, xiii, 1911) that the Fovea Centralis of 

 the eye is immensely more sensitive to red light than the outlying 

 portions of the retina^, thus a Neon lamp as a source of general 

 illumination is very disappointing, but when viewed directly 

 appears surprisingly bright. As the spinning disc of the stroboscope 

 subtends a comparatively small angle the Fovea is the only part 

 of the observer's eye used in testing, which is probably the reason 

 for the eye strain with the spark. 



Nature and Duration of the " Working Flash.'" 



If one analyses the flash of a short spectrum type Neon tube in 

 a rotating mirror it is seen to consist of two separate parts, an 

 extremely short flash followed by a flame or "arc." The first is 

 probably due to the simultaneous ionisation of the gas throughout 

 the whole length of the tube, the second to the further carriage of 

 current by the ions formed during the first. The structure of the 

 latter, which appears to consist of bright striations travelling from 

 anode to cathode at velocities of the order of that of sound in the 

 gas, is of great theoretical interest and is at present under investi- 

 gation. Discussion of its nature is needless in the present paper 

 for its duration being of the order of thousandths of a second it is 

 useless for stroboscopic work and, by the employment of a suffici- 

 ently long filament tube, it can be eliminated altogether. In a 

 lamp properly proportioned to the power of the coil in use the 

 v.'hole energy of the discharge is absorbed in the first flash. In 

 order to get some idea of the duration of this "working flash" the 

 following experiment was performed. 



A plain mirror, silvered outside to avoid double images, was 

 mounted vertically on the axis of a large centrifuge and the image 

 in this of the Neon lamp at a distance of 3 metres was observed 

 by means of a telescope with a micrometer eye-piece. Each 

 division in the micrometer subtended 4-2 x 10^^ radians and when 

 the centrifuge was running at 3500 revolutions per minute corre- 

 sponded to 5-75 X 10~' seconds. 



The lamps were run with the tuning-fork attachment used in 

 actual testing and were viewed directly and also through ground 

 glass with a V-shaped slit to be certain of getting the effect of the 



' The difference of retinal effect between red and green light can be easily ob- 

 served by looking at an ordinary luminous wrist watch in the faint red light of 

 a photographic dark room. On shaking the watch so sluggish is the green light in 

 recording its position on the retina compared with the red that the figures seem 

 to be shaken completely off the dial, giving a most curious and striking effect. 



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