Edinburgh New Philosophical Journal (1840 Edinburgh New Philosophical Journal edinburghnewphil30edin Year: 1840 - 1841 Mr Stevenson on a nem Signal-Light for Baihmys. 351 is brought into play ; and should this expectation be realized, the duration of the effect of* the light, which depends on the range, might be increased beyond what I have ventured to state. Referring to the above sketch, I would propose that the lens at L should be elevated 24 feet above the rails R R, or about 15 feet above the level of the engineman's eyes ; and that the point where the centre of the beam would interse
Edinburgh New Philosophical Journal (1840 Edinburgh New Philosophical Journal edinburghnewphil30edin Year: 1840 - 1841 Mr Stevenson on a nem Signal-Light for Baihmys. 351 is brought into play ; and should this expectation be realized, the duration of the effect of* the light, which depends on the range, might be increased beyond what I have ventured to state. Referring to the above sketch, I would propose that the lens at L should be elevated 24 feet above the rails R R, or about 15 feet above the level of the engineman's eyes ; and that the point where the centre of the beam would intersect the horizon, AC, of his vision at E, should be about 700 feet from the lens. The impulse of the light would be most ad- vantageously received at some point as near the lens as is consistent with a full effect from a flame placed in its princi- pal focus. A more remote observer would receive the rays diluted by distance ; while a nearer approach of the eye to the lens would render it necessary to adopt an ex-focal arrange- ment, so as to cause convergence of the rays. By the latter arrangement their divergence would be decreased, and the space covered by the light would be lessened not only in pro- portion to the decrease of divergence, but also to that of the cosine of the beam's inclination to the horizon. Both these circumstances would therefore combine to curtail the duration of the impression on the eye. It may naturally be expected that I should say something regarding the duration of the impulse of the light on the eye ; and upon this topic I shall, in absence of actual experiment, content myself with stating briefly the result of my calcula-
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