 It is further directed that the oil used should be vegetable oil, by preference that of the olive on account of its symbolism.Exceptionally, in consequence of poverty or some other reason, a mineral oil, like petroleum, may be employed with the bishop's permission.The language of the "Caeremoniale Episcoporum" (I, xii, 17) might easily suggest that at least in the larger churches more lamps than one should be lit, but always an odd number, that is to say, three at least before the high altar, and five before the altar of the Blessed Sacrament.