"The rate of interest acts as a link between income-value and capital-value"

This quote by Irving Fisher is referring to the relationship between the interest rate and the worth of income and capital. The interest rate is the cost of obtaining money and is identified by the supply and need of cash in the market. It acts as a link in between income-value and capital-value since it impacts the amount of cash that can be borrowed and the amount of cash that can be invested. When the interest rate is low, people are most likely to borrow money and invest in capital, which increases the worth of income and capital. Conversely, when the interest rate is high, individuals are less most likely to borrow cash and invest in capital, which reduces the value of income and capital. For that reason, the rate of interest serves as a link in between income-value and capital-value by affecting the amount of cash that can be obtained and invested.