restrictions.] In the event the board of regents aforesaid should find it advisable to issue bonds under this act [21-8-15, 21-8-16, 21-8-18 to 21-8-26 N.M. Stat. Ann.] in more than one series, or at different times, for any of the purposes aforesaid, then each series of said bonds shall be designated by the letter “A,” “B” or in some other proper designation to the end that each series shall be kept separate, and all of the requirements of this act shall apply to and be faithfully followed, done and carried out as to each of said series, provided, however, that said board of regents shall not have power to issue bonds hereunder, the aggregate interest and principal requirements for which, for any year, together with the aggregate interest and principal requirements for all outstanding bonds of such board of such institution for such year, shall exceed the amount of the income from the permanent funds and from the aforesaid income and current fund of such institution received by the state treasurer for the fiscal year next preceding the fiscal year in which any bonds of such board of such institution are authorized to be issued by resolution of the board adopted pursuant to this act.