Nichols Institute posted net income of $707,000 for the second quarter, more than doubling the $333,000 reported for the comparable period last year. Revenue rose 45% to nearly $20 million from $13.8 million a year ago. Net income for the six-month period totaled $1.5 million, which included a $210,000 gain from an accounting change reported in the first quarter. For the first half of 1987, net income was $571,000. Revenue for the most recent six months increased 47% to $38.9 million from $26.4 million. Nichols, based in San Juan Capistrano, operates clinical reference laboratories and specializes in providing diagnostic testing to clients. It also manufactures and markets kits for performing medical diagnostic tests.