embodiment the projection 21 of the bulb 1 covers the upper surface of the portions 22 of the top plate 12 which is attached to the discharge path defining member 5 therefore even if a gap is formed between the portions 22 and the inner surface of the bulb 1 due to eccentricity of the bulb 1 the projection 21 closes the gap preventing discharge therethrough further the projection 21 covers the upper edge of the top plate 12 and also determines the positioning allowing greater tolerance of eccentricity of the bulb 1 and for the dimensional precision of the top plate 12 the projection 21 further makes insertion of the discharge path defining member 5 into the bulb relatively easy the projection 21 of the bulb 1 need not necessarily be formed throughout the entire inner circumference of the bulb as in the above embodiment but may be formed so as to cover at least the upper surface of the portions 13 of the top plate 12 that contact the inner walls of the bulb fig1 shows a bulb 1 with a dome shaped top portion 3 and a jaw like flat portion at the lower periphery of the top portion 3 with a cylindrical drum portion 31 continuous with the flat portion that is the flat portion serves the function of the aforementioned projection 21 and the drum portion 31 on the lower side of the projection 21 has a diameter greater than the diameter of the top portion 3 which is located on the upper side of the projection the bulb of this construction exhibits the same effects as the bulb illustrated in fig9 and 10 and further has an attractive appearance below.