1. A method for providing a series of analog quantities that are proportional respectively to the components of a third array that is the product of a first array of components multiplied by a second array of components in a predetermined order, comprising, directing light of intensity proportional to the first component of the first array to the light side of modulating means whose output light intensity is proportional to a known function of an electrical signal applied to it, applying to the modulating means, while the light is passing through it, a signal proportional to a function of the first component of the second array such that the intensity of the output light from the modulating means is proportional to a known function of the product of the two first components, then, after a predetermined time: directing light of intensity proportional to the second component of the first array to the input side of modulating means whose output light intensity is proportional to a known function of an electrical signal applied to it, applying to the modulating means, while the light is passing through it, a signal proportional to a function of the second component of the second array such that the intensity of the output light from the modulating means is proportional to a known function of the product of the two second components, and so on, in the same manner, and finally with the last component of the first array and the last component of the second array to provide an electrical signal that is proportional to a known function of the product of the two last components, and providing a series of output signals responsive to the sums of predetermined groups of output light intensities and proportional respectively to the components of the third array.