Quoted from "The Regulations For Preventing Collisions At Sea," as printed in Nicholls's Seamanship and Viva Voce Guide, 4th Edition, London, August 1910 (in effect until superceded in June 1912):
(edited to include only those passages germaine to this discussion)
<font color="#000066">Lights For Steam Vessels
Art. 2. A steam vessel when under way shall carry --
(a.) On or in front of the foremast..., at a height above the hull of not less than 20 feet, and if the breadth of the vessel exceeds 20 feet, then at a height above the hull not less than such breadth, so, however, that the light need not be carried at a greater height above the hull than 40 feet, a bright white light, so constructed as to show an unbroken light over an arc of the horizon of 20 points of the compass, so fixed as to throw the light 10 points on each side of the vessel, viz., from right ahead to 2 points abaft the beam on either side, and of such a character as to be visible at a distance of at least 5 miles.
(b.) On the starboard side a green light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the starboard side, and of such a character as to be visible at a distance of at least 2 miles.
(c.) On the port side a red light so constructed as to show an unbroken light over an arc of the horizon of 10 points of the compass, so fixed as to throw the light from right ahead to 2 points abaft the beam on the port side, and of such a character as to be visible at a distance of at least 2 miles.
(d.) The said green and red side lights shall be fitted with inboard screens projecting at least 3 feet forward from the light, so as to prevent these lights from being seen across the bow.
(e.) A steam vessel when under way may carry an additional white light similar in construction to the light mentioned in subdivision (a). These two lights shall be so placed in line with the keel that one shall be at least 15 feet higher than the other, and in such a position with reference to each other that the lower light shall be forward of the upper one. The vertical distance between these two lights shall be less than the horizontal distance.
Lights for Vessels Not Under Command
Art. 4
(a.) A vessel which from any accident is not under command, shall carry at the same height as the white light mentioned in Article 2 (a), where they can best be seen, and, if a steam vessel, in lieu of that light, two red lights, in a vertical line one over the other, not less than 6 feet apart, and of such a character as to be visible all round the horizon at a distance of at least 2 miles.
Notice that there is no provision in the Rules for a stern light for steam vessels. The only place where such a light can be found in the 1910 rules is:
<font color="#000066">Art. 10. A vessel which is being overtaken by another shall show from her stern to such last-mentioned vessel a white light or a flare-up light. The white light required to be shown by this Article may be fixed and carried in a lantern, but in such case the lantern shall be so constructed, fitted, and screened that it shall throw an unbroken light over an arc of the horizon of 12 points of the compass, viz., for 6 points from right aft on each side of the vessel, so as to be visible at a distance of at least 1 mile. Such light shall be carried as nearly as practicable on the same level as the side lights.
Anyone schooled in the Rules understands the significance of and the difference between the specific words "shall" and "may;" therefore, I have emphasised them for ease of understanding. In Article 2 (e), above, the "shall"s in that paragraph are to be observed if the optional additional light is employed (the "shall"s follow the "may").
Nicholls's Guide was the prescribed study guide "for each grade of the Board of Trade Examinations from Second Mate to Master."
Parks